VPN loses connections after an hour

Archived from groups: comp.dcom.vpn,microsoft.public.win2000.networking (More info?)

We have a pair of (mostly) identically configured Cisco 3005 VPN
concentrators attached to ADSL lines. For the most part they work
great, but users have been reporting a problem with one of them.

After about an hour, all established connections seem to be lost. The
VPN session itself seems to still be up. The only solution is to
disconnect the VPN session and start over.

It shows up a lot while users are copying files. Everything copying
fine to a Windows share and then they get a "Network Path Not Found"
error.

I think I've tried replacing most of the cables, but nothing changes.
The other concentrator doesn't seem to ever do this. How can I fix
this?