My best guess is an incompatible antenna, somthing not made for the unit. What kind of antenna are you using, and where did you get it from? It can take a good hour or so when starting out completely from scratch (no backup battery) with the older units, with a little bad coverage. But it sounds like you have this covered.

I have purchassed a lot of stuff from ebay, and I have been pretty lucky with most of the things I buy. But once in a while, I get a dud. Hope that has not happened to you.

Walter