Pontiac Repair: Reving, grand prix gt, 3800 series ii

Question
Hi, I own a 1999 grand prix gt with the 3800 series II engine non supercharged.  When i accelerate i get a reving up and down in 3rd and fourth gear.  If i have steady acceleration it will keep reving up and down but when i take my foot off the gas or put more into it it will stop.  I do have an aftermarket air intake on the vehicle.  Do you know where the problem might lie?

Answer
Hi Scott:

You may have an issue with the transmission in 3 and 4'th gears. if the engine doesn'r act up in 1 or 2 gears the engine goes through the same rpm band in all the gears and if it only acts up in the 2 higher gears I think I'd look at a trasnmission concern here. take a llok at the fluid make sure it full and is not black or smells burnt If so That would be a good indication of a transmission issue with thie car. I don't think the cold air inatke would have much is any effect on this. has any service engine lamp come on?