Thanks dougp. I've been looking into how to do a circuit diagram so I'll try drawing one up and posting it to make things clearer.
As a test I tried moving the gearmotor C away from both the sensors of motors A & B to see if it lessened the interference, it was a marginal improvement at best. When the gearmotor C was 6' away from the sensors it would still make motors A & B stutter.
I then tried powering the troublesome gearmotor C and it's circuit from a different 12V battery. If I moved gearmotor C 6' away from motors A & B, the interference and stuttering in A & B seemed to diminish. As I moved the turning gearmotor C closer to motors A & B the interference started again. It actually seems worse when the speed of gearmotor C is higher.
Does all this suggest that it could be a combination of radiated and conducted noise causing the problems with the IR sensors?
Michael