From what I've read the solution is to flash the factory image instead, or possibly just factory reset the phone.
But be aware that even with a cleanly flashed phone, BT has major issues in 4.2. Stuttering audio, disconnects, etc. Not everyone seems to be affected by this for whatever reason, but the new BT stack is definitely no hit.
SGS3 - CyanogenMod 10.2 nightly