Cisco Routers :: VPNs Between RV042s (v3) Keep Dropping Telnet Sessions

I have recently installed four Cisco RV042 v3 VPN routers for a customer of ours to replace existing Nortel Contivity 1010 devices which were providing VPN tunnels from the customer's 3 branches to their headoffice. The original Nortel devices were working perfectly but the customer wanted some firewall rule changes and the Nortels were proving to be somewhat inflexible and incomprehensible in their configuration hence why they were replaced.
 
When installing the Cisco routers I configured the VPN settings to match the Nortel device settings so that I could swap out a branch at a time without taking the whole setup down for a day.The customer has a Unix based dumb-terminal application running on a server at headoffice that they access from their branches using terminal emulators on Windows PCs and thin client hardware devices that support vt100 terminal emulation.
 
Prior to installing the Cisco RV042's everything was working fine. Now they are using the RV042's they keep getting the sessions from their branches dropped. Both PC users and thin client users are losing sessions and it happens with active and idle sessions. I have checked the logs on the routers when users are disconnected and there is nothing logged at that time (other than my login)... I had thought maybe it was to do with tunnel renegotioations so I have set to phase 1 / phase 2 SA timeouts to 86400 & 28800 seconds respectively but this has had no effect. I had also seen somebody advised disabling 'SPI' in the firewall... I have tried this and it makes no difference.