It could probably just be your servos, if not, try changing the batteries.
If that doesn't work, probably the receiver, and if that doesn't work, I'd consider replacing all the servos and receiver.

The receiver depends on what you have. Mine happens to be a 2.4GHz receiver along with the DSM2C Radio.

http://www.redcatracing.com/RC-Cars/...CR-2C-Reciever

or

http://www.redcatracing.com/RC-Cars/Redcat-Parts/RCR-2C

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