1990 Volvo 740 GL Problems?

OK, the car has a 2.3 ltr 4 cyl. The power is drastically lagging, there is a lot of hesitation specifically in the low rpm range when you chomp on it. It barely gets up steep hills. I've changed the plugs, muffler, resonator. And I know the cat needs to go next. Could this be from the fuel filter? Or can this have something to do with the pump? I'm a good mechanic and I can do all the work myself, but whats the cause of this?