Internet connection Loss

This is a weird one for me....
Internet connection loss.
Doesn't sound weird, I know, but here are the details thus far.

2 or three times a day.
No particular schedule, always seems to be different times.
Internet connection will be lost completely.
It will stay down for 5 min to 1 hour. Again, randomly.

While internet is down all network resources are still accessible.

No found spyware or virus on machine. I have done scans with multiple
software packages. All say 'it's clean'.
Windows XP SP3, IE6.

The machine is on a domain network with 15+ other machines it is the ONLY
one having this issue.