there are several things that could be, but all fairly easy to check:
spark plugs: check the color of the plugs, should be light brown and dry
Air Filter: dirty, not properly mounted etc.
Air Leak: the rubber boots in your intake tract can crack
Ingition coils/wires: not sure how to properly check those
O2 sensor: not sure how to check it either, i think it should throw and engine code when it gets real bad, but mine didnt trip and replacing it fixed the problem
if you havent changed plugs in a while and ther air filter you might as well just give her a tune-up and not even guess about the state of the current ones.