Losing network connection?

I have a 3Com wireless modem router model 3CRWDR100A-72 (most recent
firmware) hard wired to Gigabyte GA-K8NSC-939 (nForce3 250Gb chipset) with
onboard Marvell Yukon 88E8001/8003/8010 PCI Gigabit Ethernet Controller.
Using most recent chipset drivers and re-installed the Ethernet Controller
drivers a number of times. Also turned off Power Saving on same.
At least once a night I will lose connection from PC to router, usually
after PC has been idle for a while. "Repair" on the connection doesn't
work, it get's hung up on "acquiring network address". I cannot communicate
with the router set up page at this time either.
Have also tried deleting Ethernet Controller from Device Manager and letting
Windows re-install it.
Laptop connected wirelessly to same router continues working OK.
Re-setting the router doesn't work, the only thing which does is restarting
the PC then it will likely be OK until the next night.
The problem appears to be in the PC but is it hardware, ie: the LAN
controller/motherboard or is it Windows? Using XP Pro/SP2.
Have also considered it could be the firewall, I'm using the free version of
Comodo with the router firewall set at medium and XP firewall turned off.
Should I try re-installing Comodo or even try Zone Alarm instead?
Seems odd to me that it only seems to happen at night though not at a
particular time.
Any suggestions appreciated.

--
Kenny Cargill