TA612V droping incoming call bug

I looking for some help with my Netgear TA612V with open source V1.2_73.

My setup is using Sipgate for call and Virgin Media as internet provider.
The problem I have found is after awhile I cannot receive incoming calls,
the problem is random and will recover from time to time!

I also have a second TA612V but with the Sipgate firmware V1.2_71 which
works without this problem and as far as I can tell the setup is the same.
Any ideas?

My setup:

Region: GB
SIP Proxy: sipgate.co.uk
SIP Control port: 5060
SIP local port: 5083
SIP Message TOS Value: 68
Starting RTP Port: 5004
RTP Data TOS Value: b8