Cisco 1720 Router and and 2950 Switches

We have a Cisco 1720 router, and 3 2950 switches. About 55 users. We
have a full T1 as well. Everything has been working great, about a
year since any real changes and no changes at all (that we are aware
of ) by any users or people here recently. Not software, hardware,
etc...

The problem, after about 10 minutes, we lose our internet connection
on all PCs. The local network runs great and gets fairly heavy use,
but no problem, not the first error, etc... If we unplug power from
either the router or the switches, we are back up for around another
10 minutes. It's not consistently 10 minutes, but it's always a very
short time, around that time period.

We put a hardware sniffer on the network, found a few minor problems
and fixed them, but no noticeable difference, and we still have the
lost internet connection problem. The ISP has been out several times,
tested their router, said it is fine. We also hooked up one pc to the
back of the router (all by itself), and the problem did not occur
(tested for about an hour).

I will have to get the configuration, but any ideas on where to start
looking, or has anyone seen something like this?

Thanks for any ideas, even something "crazy" might help at this
point... Been going on for too long already....

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