1991 Toyota Corolla starting problems

I have a 1991 Toyota Corolla wich run great but sometimes I have starting pboblems in special when is hot outside.I drive the car in the morning and later won't start.the engine turns,buttery good,replace the distributor wires and distributor twice already.Every time when I replace distibutor, car runs good for few months and then again...What is the problem? please help.