I have a 94 Chevy 1/2 ton 4 wheel drive pick-up with a 5.7 liter motor and it runs great. Most of the time. This truck has about 130,000 miles on it. As I said, it runs great, until I start up a hill and lug the motor. Then it will start with a stumble and does this all the way up the hill until it shifts down to another gear. The stumbles, a miss to me, are about 2 to 3 seconds apart until it shifts. It has, also, done this a couple times while stopped at red light. But, at the stop light, the miss or stumble, is about 5 to 10 seconds apart. No codes in the truck computer. So, I'm lost with what to look for. Any ideas? Also need to add, the motor has TBI and I added some injector cleaner to the gas tank with no help there.