[RyanN]

Per everyone else's thoughts, one CIM per side on a tank drive is probably your issue.

Each CIM, as Al and other have stated, stalls at 131A. Two motors at stall is 262A.

Our typical battery has an internal resistance of 0.08 Ohms. Say a fully charged battery is 12.8V.

At 262A, you'll see a calculated (not real) voltage drop of 20.96V from the battery. Given the battery voltage is 12.8V, and full stall on two motors will drop the voltage, if given the opportunity, 20.96V, there is a very good chance that the stalling is your issue.

However, I won't stop there...

Check all of your big 8AWG or 6AWG wiring from the Anderson connector to the power distribution board. Are all of the crimps well done? Give them a good, firm tug. Are all of the bolts and nuts that fasten those ring terminals on tight? Don't overtighten (break) them, but make sure they don't move left and right if you try to move them. What we are looking for here is resistance through all of your connection points. Are any of the Anderson PowerPole connector you are using charred up on the inside from people testing motors with them? That'll cause some resistance.

Adding an additional CIM will likely NOT fix your issue. You need to figure out a way to gear it down (higher gear ratio). You can try a different gearbox, add a planetary gear set with a CIM output, or change the pulley's you are using.