I'm having this prob with my 90 SSE as we speak...er write this..... anyways, for me it'* the crank sensor. It won't restart after reaching operating temps unless it sits for approx an hour but it runs great when it does run. It'* a cheap part to get and only takes an hour to fix....I'm just glad a mechanic friend was able to do a diagnostic for me as opposed to spending $100 + at the the dealership. Get some one with a scan tool to check it out...it'll save alot of guess work or you could borrow the gas and matches i was going to use on mine (not recommended for use in residential areas)
Doug