One other thing: check for a clogged air valve in the fuel cap. In a closed fuel system, the gas cap has a one-way valve in it. It is not supposed to let vapors and air out, but should admit air if the tank goes below atmospheric pressure. I had this problem once on a borrowed truck. Someone had dropped the gas cap in the mud and stepped on it. The vent was clogged with dried clay.

Another way to develop this problem is a wrong cap on the fuel tank.

I don't know if your truck can sense a loose gas cap, but later systems do have this capability and these will run terribly until the gas cap is tightened. If the gas cap vents air both ways, in or out, it will trigger the loose gas cap feature (on systems that have that feature).

hey sorry haven't looked up here in while, but no it is to old to sense a loose gas cap. But I fixed the jumping problem, new distributor.

But I have a new problem it runs fine at first start, driving and everything, but after a good drive down the road, bout 15 to 20 miles, it runs like hell and cuts off some times and I checked the fuel cap it works like a new one. do you think it might be the fuel pump

I totally agree on the diagnostics or a young kid just out of mechanic school.

It could be the computer. There could be a glitch in it. Tough to find on your own.

My 79 Toyota had that. The "kid" knew what it was. I didn't even know it HAD a computer. A new comp fixt it ($225 in 1985) but the problem came back several years later (about 1991) so therefore there was a flaw in that comp design. Since I then knew what the problem was I was able to cut the affected component out of the computer circuit and run it independently with a switch on the dash ever since. Try to sell that to a smog control check station.

In my case it was the anti backfire (or anti dieseling) (or shut down) valve in the carburetor. The comp would actuate it sporadically etc. I don't spose your heap has a carburetor but you can see the difficulty of pinning that down, I had to rig up an indicator light in the cab to establish that it was indeed that valve that was being actuated thereby causing the sporadic running.

I've replaced the ecm and the same thing the only thing that I haven't changed is the 02 sincere and fuel pump, and I know it's not the 02 that's making it do what it's doing. It acks like it is starving for fuel or spark. And doesn't do it at first start up.

Back in the era of '91, GM had a problem with the timing sensor in the distributor. Instead of points, it used a hall effect switch to sense the shaft rotation of the distributor and triggered ignition firing. (This item maybe referred to as the "cam position" sensor.) Some of these switches failed by becoming too sensitive to temperature. When cool, they worked, but became erratic when hot. It might stop working, or worse yet, trigger randomly. I had one that would either cut out the ignition or fire much too early causing the engine to buck and knock.

I traced my problem to the temperature of the timing sensor. This sensor was in the bowl of the distributor. As long as there was adequate air flow around the distributor, it kept cool enough to operate. But get in stop and go traffic, pull a hill, require extra engine load on a deep snow covered street below 35 mph, or sit too long at a stop light, it would fail. As soon as the bowl of the distributor dropped a few degrees, it would work again. This was a maddening problem to chase down. The solution was to replace the sensor with a good one.

Found the problem and fixed it, I thought the egr valve was replaced but when I talked to my friend that was supposed to do it for me, found out that he didn't replace it and kept the money for the part for something they wanted. So yea replaced the egr my self fixed it.