Ok I have a toyota corolla AE91 ... It starts it runs for 20 minutes or so then it stalls. The engine cranks but does not fully start. It only restarts when the car has cooled down. My mechanic changed the distributor coil, yet the problem still occurs. Was wondering if bad relays could cause this problem or what other explanation could be given for such occurence.

One possibility is that the engine coolant temperature sensor is faulty and is stuck in full cold mode. Lots of fuel is scheduled for cold starts, however, as the engine warms up, it needs less fuel. Too much fuel when warm and the engine floods out or riches out. The engine coolant temp sensor is telling the computer that the engine is ALWAYS cold even when it is warm. So, too much fuel is being sent once the engine is warm.