I had the same problem with a CG5-ASGT. It wound up getting a new motor board. At first it would run away in dec, other days in ra, then in both.
There have been various problems with Celestron mounts running away. Most of them appear to be related to the mount cover screws not being properly insulated from the processor boards, causing a random short.
Try googling for:
Celestron 6SE run away
You might find the instructions on now to disassemble, insulate and reassemble.
Or if it's new enough you can fault it under warranty.