Poor running 1988 Bronco II

I just bought a 88 B2 with only 89,000 miles (confirmed with carfax). Drove it from San Francisco to Salt Lake City and it ran perfectly until the last few miles. I started up one of the canyons outside Salt Lake City and the engine quit. I had it towed to a Ford dealer. They replaced the in tank fuel pump and fuel filter. The car runs now but when driving up the canyon and around Salt Lake periodically the car goes through periods of loosing power then as if a switch is turned the car runs as it should. Then this cycle will start up again a short time after, loss of power then normal. The loss of power is more dramatic the higher up the canyon. If I hold the throttle to the floor it seems to run better. If I pull the car to the side of the road during the rough running time a few moments will pass and then like a switch is thrown the idle smooths out to normal. The same rough running happens going down hill. Salt Lake is about 4,600 the canyon I have been driving up goes to 9,600 in about 6 miles. Any suggestions will be greatly appreciated. Steve