1992 Buick Lesabre mystery intermittent rev problem happens

Engine Mechanical problem
1992 Buick Lesabre 6 cyl Front Wheel Drive Automatic 99k miles

My 92 LeSabre (3800 V6) is in generally excellent condition, running well and getting very good fuel economy (27mpg highway), but has developed a problem similar to one I've seen posted in various forums, but never really answered: when the engine gets unusually hot for fifteen minutes or more and reads a tick or two higher on the temperature gauge for a while (typically in stop-and-go traffic, after a jam, or on a long steep hill-- but on a long highway drive it never happens), it will suddenly not rev above a certain level, usually either around 2800 or 2000rpm, but one or the other, not fluctuating during an episode.

When the engine tries to rev above this, it cuts out as if it's getting no fuel, but then recovers immmediately. Pressed very hard, it hesitates badly and once even stalled, though it started right back up in neutral. The problem persists even if you stop and restart the engine, but resets if I let the car cool for fifteen minutes before restarting. Occasionally after this has been happening the car will not restart at all for about 15 minutes, though this may be an unrelated problem. Normally the starter and battery work fine. But without being stopped and cooled, the car will continue to do this indefinitely, even if it gets a long stretch of highway and cools back down to normal operating temperature.

No codes the two times I've checked, though haven't checked recently and the problem is getting worse, either over time or in the summer heat. I've had the crank sensor replaced since this started, but it seems only to have cleared up a possibly unrelated problem of intermittent stalling from a stop light. New plugs, wires, coils, alternator, and water pump since the problem started as well. Fuel pressure is good (though never tested when the problem occurs) and filter is clean.

Next step? ECM? MAF sensor? Potential EGR or vacuum issue I haven't noticed, or a fuel pump problem (but then why is it so intermittent, and why have others with similar complaints not fixed them with an expensive fuel pump replacement)? It seems to me it must be a sensor/computer issue since the car runs so well otherwise. Throttle position sensor? I'm stumped.

many thanks for any advice!