Interesting that I was going to post an almost identical problem as far as symptoms go with my 88 TE (198k).
I adjusted the EHA 1/4 turn in an attempt to remedy a rough start problem and ended up with your symptoms. Mainly the idle will intermitantly drop to a rough 500 rpm when the engine is warm or after it has sat in the SC sun all day. It will stall at stops intermitantly if not shifted into neutral and hot starts require the throttle. Readjusted the EHA back to it's previus setting with little change. Fuel milage has gone down also, I assume the mixture needs adjustment. I plan to check the duty cycle this week. Any other ideas out there?

Thakns in advance.

EJC