That 2.5L 4-tech has a distributor, correct? Have you checked the distributor cap when cold for any signs of moisture? When's the last time you've performed a tune up on that car? Even back in '89 GM had some reasonably good diagnostics and supported datastream. If you have a professional GM OBD-I scan tool you may be able to glean some information from monitoring sensor values when cold versus when the power returns.
-Rod