Becker Transponder E10 error message

I've recently bought a glider that has an older Becker transponder(mode c), and every time I turn it on it invariably displays the E10 error code. In reading past reports of this issue(2006/2008) on the net this seemed to be a fairly common occurence, with no real solution. Becker says its a transmission fault involving the antenna and or ground plane. My local avionics shop tested it for the two-year cert, everything checked out OK, than at the end of the check the E10 pops up. We've tried adding a short piece of coax at the antenna end, as well as insuring the ground plane is truly grounded. So far nothing seems to rid the error message. My avionic tech here seems to think it was a design flaw in the unit, and I'm very tempted to just scrap it and move on. Is there anyone out there who has successfully dealt with this issued?
Thanks
SC