I am using the same device as yours & was faced with similar problem once. It was due to my antenna wire close to breakage with a portion inside the receiver dangling. But not quite sure if you are facing the same cause. Anyway, you may want to check the followings:
1. Wiring harness between batts & receiver, receiver & antenna. The wires should be in good cond. & not rust / rot / close to breakage at any point of it.
2. Try with the batt. failsafe & freq. failsafe function off. This will shortlist the problems.
Hope these help, cheers!