1995 Pontiac Bonneville 3.8 intermittent hard starting problem

1995 Pontiac Bonneville 3.8 mileage: 75,000. The car has an intermittent hard starting problem, particularly in hot weather. The engine cranks, however there is no spark. The dealer replaced the ECM, however the problem still occurs. The car will always start after sitting for about five minutes. Crank sensor magnets have been checked and were okay. On occasion the car has stalled while idling at a stoplight, however both times it immediately restarted.