t28 phones lagged and unreachable after firmware upgrade

[bsutton]

OK, i've not got to the logs yet but some more information.
As mentioned we have rolled back to the older firmware (yealink_T28.2.70.0.140.rom).
The system has been offline over the xmas period (we failed back to the customers old system) so we have only just got the handsets back online.
We are still however seeing some handsets become unreachable.
We currently have asterisk's sip debug options on and I'm seeing some interesting logs which to some extent line up with the handsets going unreachable:

Code:

[Jan  7 00:48:05] NOTICE[1990] chan_sip.c: Peer '110' is now UNREACHABLE!  Last qualify: 18
[Jan  7 00:48:15] NOTICE[1990] chan_sip.c: Peer '110' is now Reachable. (16ms / 2000ms)
[Jan  7 08:01:52] NOTICE[1990] chan_sip.c: Peer '119' is now UNREACHABLE!  Last qualify: 17
[Jan  7 08:01:52] NOTICE[1990] chan_sip.c: Correct auth, but based on stale nonce received from '"119" <sip:119@noojee.pmdomain.local>;tag=1976633730'
[Jan  7 08:01:52] NOTICE[1990] chan_sip.c: Correct auth, but based on stale nonce received from '"104" <sip:104@noojee.pmdomain.local>;tag=1507655074'
[Jan  7 08:01:58] NOTICE[1990] chan_sip.c: Peer '102' is now UNREACHABLE!  Last qualify: 22
[Jan  7 08:01:58] NOTICE[1990] chan_sip.c: Correct auth, but based on stale nonce received from '"102" <sip:102@noojee.pmdomain.local>;tag=1138441911'
[Jan  7 08:01:58] NOTICE[1990] chan_sip.c: Correct auth, but based on stale nonce received from '"102" <sip:102@noojee.pmdomain.local>;tag=1138441911'
[Jan  7 08:02:02] NOTICE[1990] chan_sip.c: Peer '119' is now Reachable. (16ms / 2000ms)
[Jan  7 08:02:08] NOTICE[1990] chan_sip.c: Peer '102' is now Reachable. (19ms / 2000ms)
[Jan  7 08:02:34] NOTICE[1990] chan_sip.c: Peer '134' is now UNREACHABLE!  Last qualify: 18
[Jan  7 08:02:34] NOTICE[1990] chan_sip.c: Correct auth, but based on stale nonce received from '"134" <sip:134@noojee.pmdomain.local>;tag=416868363'
[Jan  7 08:02:34] NOTICE[1990] chan_sip.c: Correct auth, but based on stale nonce received from '"134" <sip:134@noojee.pmdomain.local>;tag=416868363'
[Jan  7 08:02:44] NOTICE[1990] chan_sip.c: Peer '134' is now Reachable. (29ms / 2000ms)

You can see from above we are often seeing a 'nonce' warning after which the handset becomes unreachable.
Having said that its not consistent as you can see that handsets sometimes become unreachable even if we don't see the nonce warning.