Garrett's service is excellent, but they aren't generally very verbose in letting you know what the issue was, just a parts list in my experience.
Alons: If you've carefully checked the connections (!), ruled out EMI interference by testing at another site and trying all 4 frequencies, done a factory reset (by holding the mode button for 5 sec), made sure the cable's first wrap is over the top of the shaft and it isn't too loose/wiggly, fresh batteries properly installed, etc., it might need a trip to the Garrett 'spa'. The only time I had those symptoms was when I had a sneaky hairline crack in the coil's epoxy. It would be fine until I dunked it and then, bonkers. After a day of drying out, it was fine again. Doesn't sound like that was thinlizzie's problem, but mentioning just in case it might be yours...