Oldsmobile doesnt start on hot engine

I have an Oldsmobile 1990 cutlas sierra SL, has 187000 miles on it.
it has a peculiar problem - the engine starts on a cool engine (even in snow days I never had a problem)
but now in summer days the vehicle troubles while starting worse still if I drive for sometime and stop somewhere (after a break of 5 mins or so)
the vehicle cranks and stops immediately, or just refuses to start at all - after waiting for the engine to cool down it will start back immediately
can someone tell me whats wrong with this car