Since you've upgraded to 775 motors -- are you using a spacer around the snout of the motor (like the stock one have) to ensure they're held in place by more than just the screws? I discussed this here:
Mltoys sells one specifically for the Traxxis -- I don't know what the hub dimension is on those. If they have the smaller style (typically 15mm) then the 1/2" PVC pipe won't work. If it's larger, like the Banebots and Nichibo 7013Fs I have, then the PVC pipe can be drilled out to work well. I've also taken the rings off the stock 550 motors and drilled those out to fit the larger motors.
Having that ensures that any force on the motor (from it pushing on the gear) gets transmitted into the gearbox at the stronger mold point where the stock motor's ring pushes. WIthout it you're just held in place by the screws and although they're a tight fit in the stock box, the original design didn't put the force there. I don't know how much that flexes (plus it's further from the actual gear interface.
F-150 - 24-36V with homebrew 100A variable-speed motor controller, 4x 775 motors (no, it's NOT all-wheel-drive--look HERE
for more details!)
Pink Rocket Princess Mustang - 15mph, 36V, 4x 775 motors, homebrew PWM controller rev 2....
Hurricane - 24V w/50A step-down controller, 775 motors