Our 90 nissan sentra keeps losing power, stalls at stops, and wont stay started.

What is your question?

My fiance would be driving up a hill to work, or go from a stop and when he gets to a steady pace, the car will sound like hes pressing and releasing the gas over and over, then it'll stop. He will start the car to go somewhere, and the car just stalls and keeps turning off, he has been trying to keep it running for about 10 minutes now and it wont stay on. Also, at an idle, hell press the gas and it sounds like a putty back firing noise kind of (hard to explain). We have NO idea what it could be. We checked spark plugs, hes going to clean the carb out, and see what happens, but from my description, what could it be?!?!?!