rpm's fluxuating problem

My grilfriend has a 99 chevy cavailer 2.2L manual transmission. Lately while driving, the car feels like it is "kicking" and the rpm's fluxuate up and down (not alot) It never does it while is is stoped and running. It started out doing it only once or twice a day now it seems to be doing it constintly starting today. No check engine light has come on. The car only has around 48,000 miles on it. I put some fuel injector cleaner in it but I dont know if that will help or not. Any other suggestions?

sounds like a plug wire possibly. check engine lights come on only if one of your sensors either detects a problem with a monitored system or if it is faulty so you won't always get a light if it's something like a spark plug is fouling or your fuel filter is plugged etc etc. depending on which 2.2 you have it may not even have plug wires but I believe that your does. if one of them is bad it would idle fine for the most part but when you put the car under a load, it would buck and jerk due to a misfire. hope it helps, and I hope it's something that simple!