Strange Router reset problem

[samsat]

We have been getting a lot of ADSL line drop outs.
Telkom's service has been very good in helping to try and trace the fault.
After much work by Telkom the problem remained and I decided to replace the router. No difference.

Here's the setup. The router is plugged into our 3Com switch which provides access to our users at work.

When the line drops, I cant even ping or access the router's IP address. It appears that some ADSL line fault is causing the router to reset. This has happened on both routers (Billion 7402R2 and the new Billion 7402G).
After a short period I can then access the router and watch as the connection is re-established.

Has anyone come across this before?

 

[samsat]

UPDATE.
I have been monitoring the router over the weekend when almost all the PC's on the network are switched off.
Now the router is not resetting. It has been up for a full 24hours whereas during the week it would reset every 20 minutes or so.
It appears now that one of the network PC's could be causing the router to reset and the ADSL line is dropping as a result of this.

Can anyone of your network experts give me some pointers as to what's going on here?