[BrianG]

If it appears the BEC is getting weaker, then yes, I'd send it in. Even if you disable the BEC to keep it from powering servos/receiver, it still powers part of the ESC itself and a dodgy BEC can cause weird things to happen.

Once you get a fully functioning ESC replacement, I would disable the BEC (simply remove the red wire from the throttle cable) and use an external BEC to help keep the problem from happening again.

IIRC, the sidewinder uses a linear BEC, which can thermal when running at 3s voltages. An external BEC is almost always switching, and they are more efficient so there is less chance of thermalling with increased voltages (assuming the servo isn't pulling more current than the BEC current rating).

Also, you might want to check to make sure the servo is not worn mechanically, and that there is nothing resisting the servo action (like dirty/worn steering linkages/parts). Any of those would cause more drag, which causes the servo to draw more current and stresses the BEC more.