My NTL D5100 cordless phone keeps flashing AUTO?

My NTL D5100 cordless phone keeps flashing AUTO. When it does this, I am unable to make or receive calls. It randomly fixes itself.

When the D5100 base station flashes AUTO, the registered D505 handsets also flash AUTO. I have tried switching the base station off and on again, but this doesn't fix the problem. When it decides to work correctly, it only works for a short period of time. I thought this might mean the batteries in the handset were low - but I've replaced these and that doesn't seem to make any difference.

Any ideas how I can fix this?