1994 Chevy Truck loses power

Truck runs good except when I am going down the highway and push on the accelerator to go up a hill etc.I get a short quick boost of power but then it bogs and stays at the same speed.I have to push accelerator half way to the floor to gain any speed at all. I have checked fuel pressure, TPS sensor, timing, replaced MAP sensor, replaced distributor with a used one(dont know if there is anything wrong with it), checked fuel injector screens at the base of the fuel injectors. It acts like its not getting fuel or the injectors are opening and then closing back up. Or timing not right. At my witts end! Please advise!