The battery is the one in the drawing, a 12V 22AH SLA battery. The small gearmotor C which seems to cause the problem when it is turned on has a stall current of 3.8A. With gearmotor C turned off at the switch or it's pot turned right down to 0V the stuttering on motors A & B (each controlled via PID & the two sensors) goes away.
Apologies the drawing isn't clear, I'm not sure I know how to do a circuit diagram.
Let me know if there is any other information I can give you which might help.
Thanks,
Michael