1990 Chevrolet Lumina Engine stalls intermittently

This is pretty common on any brand of car. What's not common is the four months between failures. Most commonly the crankshaft position sensor or the camshaft position sensor start to fail by becoming heat-sensitive, then they work again after they cool down for about an hour. The clue here is when it won't start, there won't be any spark.

GM used a few different systems and I don't know which of these applies to your engine. On some, when the camshaft position sensor fails, the engine will continue running until you stop it, then it likely won't restart. The Engine Computer doesn't know which piston is coming up on the compression stroke, so it takes a guess. With a V-6 engine, it has a 33 percent chance of guessing right and starting. It will run, but not at its best. If it doesn't start, simply turning the ignition switch from "run" to "crank" repeatedly won't solve anything. The computer just keeps on calculating the next cylinder to fire and it will continue to be wrong. You have to turn the ignition switch all the way off, then try cranking the engine again. Eventually it will guess right and the engine will start.

Some engines simply won't run when the signal is missing from one of those sensors.

The problem with the fault codes is sometimes they take too long for the problem to be detected. The engine comes to a complete stop too soon, then both signals are missing, and that's a normal condition for an engine that's not running.

What you may be able to do is connect a scanner to view live data, but of course this has to be done when the problem is occurring. The Chrysler scanner shows both sensors and lists their signals as "no" or "present" while you're cranking the engine. It works on all other brands of cars too, but only back to '96 models. I don't know if the older GM scanners show those signals.

There's three things you can consider, and none of them are ideal. One is to replace the crankshaft position sensor and hope that solves the problem. We hate just throwing random parts at a problem, but that may be less expensive than having a mechanic waste a lot of time on a problem that won't act up. Next is to find a mechanic who will let you take his scanner with record capabilities. When the problem occurs, you press the "record" button, but you have to be quick. Because the data travels through the scanner's memory, the recording actually starts a few seconds before you pressed the button. Later, that data can be reviewed slowly to see what changed. The third option is to keep driving until the failure is permanent and can be diagnosed. Unfortunately, any of those options could leave you stranded in the middle of nowhere, ... On a dark, moonless night, ... With nothing to do but wait.