[Al Skierkiewicz]

David,
A Vbus fault could be caused by a bad crimp, bad breaker or improperly terminated wiring in the PD. It is simply an indication that when you draw current, the voltage drops feeding the Jag. You might get the same fault with a motor that is drawing a lot of current but not enough to throw a high current trip.
Check every inch of the wire between the PD and the Jag that is faulting looking for shorts, bad crimps, whiskers, loose hardware. Try spinning the motor by hand and see if there is a lot of resistance to your spin. It could be a bad motor, or improperly mounted motor on the transmission. If it is a drive motor, do you get the same results with the wheels off the floor?