Netopia 3347NWG-ENT/3387WG-ENT routers

Hello there!
We have more than 60 netopia modem/routers models 3347NWG-ENT and 3387WG-ENT , all around our remote locations connected to the main office whith PPP connection from DSL services provider under PPPoE or Bridged encapsulation, and VPN router-to-router the office firewall. However, some these routers keep dropping the internet signal so often, and sometimes they require a hard reboot to dial to ISP again.
So I have set up a scheduled connection such there won’t be any timeout in case of TCP inactivity…but still have problems. Now I found out that some those routers are creating Tx Errors, and such errors are related on MTU ( I use SNMP to pull out the ifFragFails value).
Any ideas on how to keep these routers up at all the time?