scubaEVO

Lately I've noticed that after driving for a little while that when I shift the rpms hang/jump up about 500 rpms and while coasting the idle varies between 1k and 1.5k. Once I have come to a stop the idle varies between 1.5k and 2k sometimes. When I let the car sit and cool for awhile it seems to go away but, then shortly returns. Even while trying to maintain a constant speed the rpms pulsate causing the car to rock back and forth as if I'm tapping the throttle on and off. Any suggestions on what might be causing this issue? Could it be as simple as just a bad sensor? If so, which one? Thanks in advance for any help to fix the evo...