1991gxe

hello i noticed when i'm driving and step on the gas i hear a clunking noise coming form the engine. i usually hear this when i'm driving uphill. it feel like the car loses power as well. but the nooise sometime appears in daily driving say if i was to floor the pedal or so. somedays i hear some days i don't. what is that noise and what is casuing it. is there anything i can do to get rid of it. i use mobil 1 oil and run it on mobil 93 octane.