B_H,
It sounds like you are very sharp in this regard and have done everything possible to remedy the problem.
If you did everything right, what didn't you do? Do you think that you changed something and the problem really did go away for a day, or was it wishful thinking.
Your fix-it list truly covers everything possible short of a hanging valve. I would try to dial in on the problem more. Find out if you can recreate it at will, and note what conditions bring it on. This will take lots of things off your "what is it" list.
'Under load' means low vacuum, power valve, higher combustion pressure with resulting spark loss. Maybe try looking in these areas.
If nothing else, dump a can of Berryman Chemtool in your gas tank and see what happens. Good luck!
Stuart
