95 Jimmy engine issue when hot
 

All right, this one's got me scratching my head.

1995 Jimmy, starts right up fine and runs perfectly. I generally have no problems with it, all the way up to operating temperature; but therein lies the problem. After extended trips - let's say around 20 to 30 minutes or more in length - it will run fine all the way, until I turn off the engine and it starts to heat up past operating temp. If I start the engine again within only a few minutes it will start easily and the gauge drops back to OT immediately, but as soon as the tach starts to approach 2000 something weird happens - the engine seems to cut out and recatch, cut out and recatch, over and over - not as if the engine is bogging down or losing power, but more almost as if the timing is suddenly completely wrong. At gas-off idle it seems to run just fine, and USUALLY if I slowly accelerate with a mind to keeping it below 2000 RPM I can proceed as normal - so long as I get it up to a decent speed and can consistently stay there (i.e, highway). However, as soon as I slow down again, the problem can return.

Other times, I won't even be able to do the slow-acceleration trick; from engine restart, the timing is so bad that I can't force the vehicle to more than around 10 mph - with a freely running but badly stuttering engine.

In either case, when the engine is stopped for longer than around 10 to 15 minutes, I can usually start up and proceed as if everything is fine - again, as long as I can highway it all the way home with no more than a few occasional traffic light stops.

There is never a check engine light during any of this. When allowed to cool down completely, no symptoms return until after the next long drive.

It's hard to exactly describe the behavior of the engine, so I may not have done the best above. However, it does not "feel" like a fuel delivery issue, it feels like some kind of electrical issue. My own suspicion is that an electrical part is acting up when it's getting hot. Tapping on the most obvious culprits (air flow sensor, pressure sensor, idle controller) don't have any effect while the engine is running (trying to run the engine with one of these components unplugged of course results in various fun but expected effects - I had to try anyway For Science!). EGR is fine, but I don't think it would be responsible for those symptoms if it were clogged anyway.

Any ideas?

So, nobody can think of what might be causing this issue?