1990 Pontiac 6000 run

1990 Pontiac 6000 6 cyl Front Wheel Drive 179000 miles
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MY PONTIAC STARTS AND RUNS FINE FOR ABOUT 20 MINUTES AND THEN JUST SHUTS DOWN AND WON'T START UNLESS YOU WAIT ABOUT AN HOUR OR SO. i REPLACED THE IGNITION CONTROL MODULE BUT NO HELP. ANY SUGGESTIONS WOULD BE APPRECIATED.

THANKS,
GARY