Tape a meter up to the top of it and drive around and watch the meter!
Or maybe even make use of one of your analog or comparator ports, kill the motors when the voltage gets too low, use a seperate voltage reference, blink and LED when the volts are good, no blink when volts drop off, etc.etc...(which is why I said add a load of extra vias even if you don't use them, makes it a piece of cake to add this stuff in there)
Batteries - that's what I'm thinking. If I remember right, your motors are driven off a seperate motor driver chip with a mind of it's own and probably doesn't need a full 5v to operate.



but if the problem is a magnetic one you will need something with a high magnetic permeability. Hold a compass near the motors when they are running and see what happens. Use one that you will not care if it gets reverse polarized. 

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