Recently bought a 91 F250 351 W with a manual transmission. The truck has 138,000 miles on it. The truck runs great until you try to push the RPMs high quickly (going up steep hills or quicker acceleration) in 3rd, 4th, or 5th gear. I'll be accelerating and then the I'll lose power completely but it will come back in a second or if I shift up or down the power will come back. Replaced the fuel filter but didn't help. Thought it might be the throttle sensor since that is a common issue but it idles perfectly (still could be it I guess). New and learning to take care of this old truck. Any advice on diagnosing the issue would be helpful.

Thanks