BT Synergy 3105 handset (intermittent) reset / re-registration?

From time to time, by 3105 handset beeps once and then the display flashes and for a short time (around 30 seconds), it is not possible to obtain a dialing tone. Then the phone makes plays a series of short beeps (I believe this normally signifies a handset >base-station registration), the display returns to normal and the handset can be used again. This happens both when the handset is on the base unit or detached. The battery display shows 'fully charged'.
I have tried setting alternative Base station 'numbers' (1-4), but the handset continues to de/re-register once every 2-3 days. Annoying !