[merc81]

I would check out the MAP sensor. Try to find one as a donor before purchasing one as they cost $50 bucks and you want to be sure thats the issue. MAP sensor failure will cause poor idle but doesn't effect engine after 1000 rpm or so. It will make it hard to start.
Almost any GM car will have a MAP senor you can 'borrow' to test. Its a small, 1 inch by 3 inch by half inch thick sensor that has a 3 pin connector and a vacuum hose.
These things can fail when cold and start working when warm again.
good luck
John