[Kevin Sevcik]

I second Art's questions/suggestions. From your description of your setup, it sounds like you haven't properly matched the ratios of the CIMs and FPs. If you don't have them well matched, they'll be constantly fighting each other, and it will only get worse the faster you drive them or the more external load you put on them.

You should be striving to match the free speeds of the motors as closely as possible. If I'm reading you correctly, and your CIM free speed is 675 RPM, while your FP free speed is 120 less at 555 RPM... That's entirely too large a difference and easily explains your problems. You need to match the gear ratios much more closely than that.