What cause a 94 chevy 1500 4,3V6 not accelerate when going up hill and when trying to pass it bog downs?

I have a 94 chevy 1500 with the 4.3L V6 Auto transmission. The truck will not accelerate like something is holding it back when going up hill and when trying to pass. The engine will either mis, or bog down, or not change RPM's at all. It runs find when i first start it up and when i'm holding the brake and gas it smothers out like its not breathing and trying to cut off.