About overheating:

Possibly, the electric radiator fan is not turning on when it should. First suspect is the coolant temperature switch, the one that calls for the fan to operate.

Your crank sensor and cam position sensor codes are another matter. Without these, the car won't start, but since it does start, suspect a cam sensor failing when hot. Hopefully, John_G will chime in with his expertise.

At 265f your engine is seriously overheating. The fact that it cools down when you stop suggests first that the cooling fan is running, which you should easily be able to verify just by looking. It also suggests that you have either a coolant flow, or air flow problem as the main issue.

Question, is the plastic air dam that fits below the radiator support under the front of the car in place? This air dam forces air up through the radiator, while also helping to create a low pressure area behind the radiator in order to help force as much air as possible through the radiator. The coolant fan is not part of the problem when you are driving. By design it turns off at about 25mph, because there is more air going through the front of the car by then than the fan could ever pull.

Check for debris that could be blocking airflow through the condenser and/or radiator. Things such as leaves, cottonwood poplar "cotton", paper, cardboard etc could restrict airflow and prevent the system from cooling the engine.

From here we get into the possibilities that the engine isn't actually overheating and maybe the gage is bad. Is the computer system turning on the check engine light? If so you should see a P0114 setting. A technician with a scan tool would be watching the coolant sensor's temperature input to the PCM. A technician would also directly measure the engine temperature with an infrared thermometer. With reported problems like this damage to the water pump impeller potentially comes into play, however traditionally GM does not use plastic impellers as many of the imports do. Many imported vehicles suffer failure of the plastic impellers and that results in the pump not actually being able to pump the engine coolant fast enough to allow the engine to cool under load. But it may still cool when stopped and idling.

A thermostat frozen in one position could do this, but they are usually over replaced in practice, with the person doing the work ensuring to fill the coolant system fully, so it then appears that replacing the thermostat fixed the overheating, when in fact the system was actually low on coolant for an as yet undetermined cause.

I don't expect there to be a connection to the missfire/backfire and the overheating situation. UNLESS, the engine is not actually overheating, and there is found to be a reason that sensor inputs to the computer are being corrupted in some fashion.

I am begginning to tire of warnings lit up on the dash. It seems to be that the on-board diagnostics are rather over-zealous. Only once in the last ten years has a warning light been justified on my dashboard, and that was only down to an expired glow plug. Now every time I seem to get a full house in my new favourite game 'warning light bingo' I book the car into the shop under warranty and they automatically say first of all that it is probably a faulty sensor. There is never actually anything wrong with the car, just a bit of metal telling the ECU porky pies. Now I'm fine with this, I have a company car under warranty. But what about the poor sod who is driving my car as a jalopy in 15 years' time?

If the engine is truly overheating, that future "poor sod" won't be driving this car, at least, not with the present engine.

Its a company car - Don't they have a policy about maintence and repair? The company policies that I know about says that if you ignore all these warnings and someething bad happens, you buy the car. Surely, they have a maintence contract. Why not take it in and let the tech have a look at it and pull the codes?

John_G has listed several possibilities and all of those are trouble makers.

I'd suspect on a ten year old vehicle that the radiator is getting clogged up. If coolant isn't changed as per recommendations, this can easily happen. The fact that it doesn't overheat at idle would pretty much confirm this diagnosis. A new radiator is much cheaper than a replacement engine....There are no real alternatives to this. In the old days you could flush them or 'rod them out' but cars nowadays have such narrow coolant passages in the radiator that cleaning is an exercise in futility. I'm assuming you can hear the engine fan running...

jemdandy, it is a classic case of crying wolf. My car keeps saying over and over again that something is wrong. Each time it is proven wrong. How long do you put up with such a charade? I think he who is in charge will vote with his feet when the current fleet is due for replacement and go for a manufacturer whose models only put the red light on when something is actually wrong, as opposed to whenever there is an 'R' in the month.

"opposed to whenever there is an 'R' in the month."

That's a good one!

If everything is working well, its probably bad sensors or bad connectors in the wiring harness, particularily if the car has been operated where road salt is used. To sort out this kind of problem requires a QAULIFIED technician, one who is well aquinted with this model's circuitry, control scheme, and has the proper instruments.

What I gather from your postings is that you are getting fed up with many seemingly bogus warnings.

In the 1980s, I had a circuit problem with a GM car. The gasoline gage went out. I found that the problem was in a flat cable running to the trunk. There were several wires in the cable, and one was aluminum instesd of copper. The aluminum wire had corroded off at a connector in the trunk and it carried the signal for the gasoline gage - an easy fix. [Preliminary test procedures indicated one would have to drop the gas tank and replace the float assembly.]

This isn't the only circuit problem that I've had with GM autos.

Jemdandy, what a coincidence. you mention personal problems with mickey mouse readings on a GM car. My current model is an Opel Vectra - Opel are owned by GM. In fairness, this latest model has not been as bad as previous Vectras I have had. The first one, a top of the range 2.2 turbo, spent most of the time back in the shop, leaving me wobbling around in a 1.3 base model Mitsubishi Lancer courtesy car instead. Maybe the real reason why GM are penniless now is that they are selling cars that are rubbish? I don't hear the Koreans or Japanese going cap in hand to their governments for cash bail-outs. Rover went bust because they spent all their time on strike. And when they did make something it was crap. Just look at the Austin Allegro and Morris Marina. If GM don't buck their ideas up they will go the same way. Ford and Chrysler had better learn too, they aren't far behind GM.