Sounds like what happens to my HS-81's. When that happens, following Hiteck's instructions, I open up the darn things, carefully open the feedback pots and spray them with tuner cleaner and exercise them alot. Repeat 3 or 4 times then reassemble carefully making sure they go back together in the same orientation that they came apart.
This procedure has worked for me on 4 servos so far.

JESTER