Things I would check out in order....

Plug wires
Plugs
Clogged Injectors
Vacuum leak
Fuel Filter
Fuel Pump

If you are ruling out bad gas and it has trouble idling at a low RPM, it probably needs a tune up. Take it to a respected mechanic and have them look it over. I can tell you, I just changed my fuel filter (52k miles) and it was pure black coming out of it.