I have a 91 k5 blazer that start giving me problems after 4 hours of driving. It start on hills It start bogging down then it goes back to normal then it bogs down again ones it starts it gets worse. It start doing it on flat road to where I cant get going it like it jerking back and forth. The power goes away then it comes back it does this for a while then it starts to die. It only does this after a while of driving

 

HAd a similar thing happen in Colorado on the freeway. Hi altitude, hot, had just put an electric fuel pump under the hood where it didnt belong. Worked fine at low altitude

It got its revenge on me at 6,000 feet...could see thru the clear fuel filter there was no liquid in it. Moved the pump to near the gas tank, problem eliminated.

Measure charging system voltage, must be 14V plus.
Hook a timing light to the coil wire, tape the light on the hood or windshield. See if the light dims or drops out.

MAy have to do a running fuel pressure check too. Got air, fuel and spark, thats most of whats required.

Its also possible a U joint or differential bearing are seizing up. A U joint can be dangerous if it lets go. "After 4 hours" means the driveline is hot.
HAd U joints in a 78 Plymouth cop car that seized, the car wouldnt move!