Well, there are several occurances that could have possibly happened.

It could have recieved some interferance from another source or device. It may also be a problem with your servo's or wiring itself. Double check and make sure there are no loose wires or any moisture around the wires. They should all be properly connected. You may also have a week battery(s) in either your car or transmitter. Make sure they are fully charged for best preformance.