First off make sure you wired it correctlly. I'm gonna assum it is, so it's probablly putting to much strain on the batteries. Castle Creations say that for the 6900 and 7700 you must run 8 cell Nimh or 2 cell lipo. www.castlecreations.com/products/mamba_max.html
You have two options, you can either upgrade to lipo, which you would also need a lipo charger or you can just by the 5700 motor for probablly less than $100.
Hope this helps, good luck.