Surging motor

Hi, guys. Long time, no post. My previous 8g was toast about 4 years ago thanks to my soon to be ex wife. So I found a shell and took my running 02 motor and dropped it into an 03 shell. Since I've had it running(not driven) the motor has been surging. I'm not quite sure what it is but I have been doing some research up here but haven't come up with much to help me. And when i say surge it mean it surges in park and in gear. Are there any common issues that i should be looking at? I would love to get my 8g running again since I have a slew of parts from the previous one. Any help would be great.

Diaz
TGCNC