The Occasional Gutless Ford Pickup

I have a 1994 Ford F-150 Pickup with 55K miles on a remanufactured engine. I recently brought it to my mechanic (not a Ford Dealership) because the Pickup had no power. My mechanic found a faulty front fuel tank pump, and replaced it. Now, when I drive the Pickup, it starts to act like it did when the front fuel tank pump went bad, but only when running on the front tank when it is about 3/4 full. When I switch to the rear fuel tank, the problem goes away. I have told my mechanic that I think the front fuel tank pump is faulty, but he cannot replicate the situation. Any suggestions?

Has he measured the fuel pressure from each pump?

Unless your mechanic replaced the front tank fuel pump but failed to replace the inline fuel filter, you are almost certainly right. As already noted by Goldwing, the pressure for each pump should be tested. But you should bring the car in with the front tank 3/4 full, that being the only commonality you have noticed.