doesn't this problem manifest itself if the engine loses compression when hot? perhaps you have an apex seal groove that is expanding too much under heat. check hot compression if you havent.

 

Actually, that's a good point as well. I have rotary aviation seals, which apparently take 3000-5000 miles to fully wear in & seat properly. If things get really hot, it feels like I'm losing compression (engine braking goes from adequate to sucks). I probably should get a tester on it at some point.

This weekend, I'm redoing my coolant system (new thermostat, flush system & replace with proper mixture of coolant for hot weather). I'll see if that makes any difference.

-=Russ=-