89GMC Jimmy S15 4.3L Vortec Starts up fine and runs great for about 5-6 miles and then hesitates or stumbles on acceleration. Can accelerate if slowly stepping on gas pedal. Park truck for 1-2 hours and it will start up and run like brand new. No miss or hesitation. Cleaned EGR valve twice and installed kleen screen gasket. Replaced spark plug wires, PCV valve, fuel filter, and ran injector cleaner in gas tank three times. No help. Your help would be appreciated.