Ditto what Lee says. I've even seen cases where glitching occurs because of a servo lead being physically tied to a gyro lead - the pilot was just trying to keep everything neat by bundling the wires together, and ended up inducing noise in the servo wires. He was dumbfounded at first as to why a PCM system was glitching - he figured it should be locking out if there was interference, but the lockout only occurs if the 'interference' occurs before the incoming signal is decoded and checked for valid checksums etc.

Gordon