W52 ovpn drops after initial connection

[dpl73]

We have about 20 of these deployed in various locations. All are fine, except for one site who have 4 x w52 and 1 x t26. The w52's connect (vpn) and register for about 2-3 minutes and then the vpn drops. Normal network connectivity is maintained on other devices PC's, Servers, etc.. and the T26p maintains it's VPN connection, so it is not likely that the internet connection is dropping.

I've tried base resets, various firmware down/up-grades, new vpn.cnf's and certificates.

This site worked fine for about a month, and then there was an network issue with the Telco/ISP which has been resolved. After the WAN connection was back up, this started happening.