My 1994 Grand Prix with 238,800 miles (just short of the distance Apollo XI traveled to get there), has an odd intermittent problem: the engine violently cuts out as if someone were toggling a switch back and forth. It first started a couple of years ago (early 2006?), at which time the ignition module was replaced and I believe the TPS as well.
It began again a few months ago. Whether at speed or at idle, it would intermittently cut out. If it died altogether, it would not restart, unless, I made no attempts for 3-4 minutes, at which time it would be fine. At its worst, this had to be repeated every mile or so! Finding no codes, my mechanic got off on the idea that the engine was running cold. If my memory is correct, the O2 sensor was fine as well. It also flunked HC emissions at this time. I replaced the thermostat, it passed inspection, and lo and behold, the cut outs went away. In the last few days, it has started to reappear, worse than before.
The replaced parts were original, though the engine itself is a used one put in at about 203,000 miles after the original started running rough a couple days before hitting 200,000. Eventually, it took to shooting spark plugs out of one cylinder. The ECM is original, and does have a problem with turning on one of my fans. This was in fact bypassed several years ago (2004-05?) and the fan shorted to the frame to operate all the time. This seemed fine and caused no obvious problems. This short was disconnected prior to the reinspection in the previous paragraph. As it has been hot lately, I reconnected it, though it seems to make no difference in the appearance of the cut out problem in this round.
The first thing to check when the trouble happens is the ignition and see if there is spark getting to the plugs. There may be a intermittent power connection to the system. If that is good then the fuel system needs to be checked.