It wouldn't be able to deregistrer if it was power cycling due to bad battery or similar?
Could it be forbidden in the system to use that TG at that particular site?
I have seen this phenomenon happen when I tried to configure a system to not allow certain TG's on certain sites, and it couldn't be used due to this logical "bug" that the actual radio didn't understand it was prohibited to use the TG on the site and should roam to another site although the actual radio ID where allowed on the site.
/Ubbe