With no event codes going off that usually rules out electrical issues. If the issue isn't happening when reversing it shouldn't be the gear boxes as the SCs don't have a sperate reverse and forward gear. If the drive motors are DC there could be an issue with the brushes or armature. It could be possible the motors are grounding out to the case.
Without event codes I wouldn't be able to tell you for sure what is wrong without a test drive.