well it may sound stupid and I kinda dont even want to say this but at my field we had A guy with his own radio like cb call station , well every half hr that there on "line" they have to call in there #'s or radio call sign. Well we noticed the same thing and lost 1 plane before noticing that he was the problem. Long story short he felt like A moron and paid for the repairs to the rig and now everything is fine he keeps it off! Anyway longshot but that was our problem. good luck and keep us posted.