i wonder if any of you can help me with a very strange (and annoying!) issue

I have a 2mbit adsl connection 'managed' by a US Robotics 4 port router.
This comes into 8port ethernet switch - which lets our 4 pcs (kids etc) and
a laptop have net access
All boxes run xp/sp2

Most days, my adsl connection drops for no apparent reason sometime in the
afternoon - just for a minute or so, to resync adsl, get my (static) IP
address, then everything is fine again. It will do this every 4-5 minutes
about 5-6 times, then seem to be fine again!

I've run anti-virus, anti-spyware tools on all boxes
Checked and replaced cables
Bought a new router (was using another less known make)
Checked router config's - as far as i can
Changed all local boxes to dynamic ip's, instead of static 192.168.0.x
Checked DNS server entries on the router
etc etc etc

Also spoken to my isp and had line checks etc done - everything was 'fine'
according to them

Nothing has helped and it is driving me crazy!

If anyone has any ideas, please let me know

Many thanks