Arh man, gutted. The effort to get to this point and it might all be for nothing.
As a test, I removed the DRV8871 gearmotor C driver and used one of the IBT-2 motor drivers from A & B. With just motor A running with the PID, I turned the gearmotor C on and increased the speed with the pot - it just acted exactly the same as with the DRV8871 - motor A starts to stutter, it seems to get worse when the speed of gearmotor C is increased.
The gearmotor C is 4' from the sensors, the only thing the gearmotor C circuit shares with A & B is the same battery. I've even tried it running on another 12V battery but if you move it within a few feet of motors A & B when they're running - they start stuttering again.
Michael