I had a car with the electronic ignition pack doing that. Factory bulletin about it even.
Quite a costly thing as the dealer couldn't repair it for free unless he could experience the failure while in the shop to asscertain it was one of the bum ones.

I'm greasy under my fingernails occasionally so I just bought all new aftermarket distributor & ignition - it was crazy, coasting to the side of the road during rush hour everyday in the same general area.

That's a tuffy. Pretty hard for a mechanic to find a phantom problem like that. Could be a lot of things and good mechanics really don't like to throw parts at something and hope for the best.

Does everything quit like the radio, ac/heater fan, ect, or just the engine? Do all the idiot lights come immediately when this happens? If everything quits, it makes me suspicious of something like the ignition switch or related wiring and connections. Most of the time when a componant fails, the check engine light comes on and sets a code. I suggested the ignition switch because if that fails, the computer might think you just turned the engine off. Just an idea. If a mechanic looking at it can't see the problem, computer land people sure can't. Though it doesn't sound like it, the best thing that could happen at this point is if it would die and stay dead. Then someone could zero in on it. Good luck with it.

I'd check out the ignition module. GM has had some tricky ones in the past. It may stop working or trigger at the wrong time after the module exceeds a certain temperature, and then begin working again after cooling off a bit. Its hard to anticipate under what conditions will cause this particular sensor to fail because of many variables. Road draft and air currents under the hood affect the sensor's temperature. Some conditions where you may expect failure:
1. Creeping or sitting in slow traffic.
2. Pulling a long hill at less than 35 mph.

It works ok at cold startup. Only fails after it heats up.