On my JDM 2004 AT, If the car is stationary either when I start the car or at a stop and I start moving my RPM will go to 2000-2500 and drop 500 RPMs to like 1500-2000RPMs and go back up. (Regular driving no launch or brake launch)

What would cause this? I had a check engine light but it came off after 2 days I thought it was just because i removed my CAT so i'm straight pipe and the sensor was just messing with me.
Also the car seems to be running rich at times.