It could be almost anything.
I'd start with the easy stuff and pull a couple plugs. If they look nasty, clean and re-gap them - or buy new ones. Take a look at the distributor cap and rotor. If they look nasty, clean or replace them. How about the plug wires?
If you want to rule out fuel, then get a pressure tester and hook it up.
Fuel Pressure - Checking If the pressure tests low, then perhaps it is the fuel filter. But it could also be sediment clogging the screen on the intake in the tank. And it could also be a bad fuel pump.
There is no silver bullet for any of this. You just have to be methodical and go down the checklist and eliminate possible causes.