Losing Network Connection

I am running Windows Server 2003 in an AD domain with all
2000 and XP clients. I also have a NT 4.0 server with
exchange.

We have intermittent (once a week or so) issues with a
user losing there network connection and going off-line.
If they "Repair Network Connection" they are fine. One
user in particular seems to experience this more
frequently.

I suspect that I am too thin on my licenses, but cannot
find any error messages in event viewer to confirm. Is
their anything that I can check before pulling out the
VISA card?

Thanks
Chuck Ghastin