You may need an external BEC . I learn this from my crawler experience having high torque steering servo's and the internal BEC in the ESC not being strong enough for it. With the added extra draw from the motor and ESC to run the belts it could drop into a low voltage protection for your drive motor and ESC. And with any additional electrical add-on ( including steering servo etc...) can cause voltage drop. Anyway , just put a Castle Creations BEC between the original ESC and your steering servo or belt ESC and all should be well, they only cost $20
http://holmeshobbies.com/Castle-BEC.html