I typically get random servo movement whenever I do anything with electrical connections around my plane... e.g. plug in a glow ignitor connected to my starter battery, etc.

However this has never affected my plane in flight.

I have had servos that randomly move due to interference from the motor.

Typically these are bad servos though, and I end up replacing them, causing this to stop.

Perform a range check.

Then run up the engine with the plane restrained and have someone watch the servo movement as you get to about the same distance you were having problems.

Remember that local towers, etc. can cause interference that will NOT show up as an occupied channel.