losing Internet connection when hooked up through router

Hi does anyone have any ideas why I am getting dropped/cut outs in
broadband when connected through a Router but not when I plug the Ethernet
cable straight into the Ethernet/Network socket on PC?

More details:

Dell Vostro PC - windows xp professional.

D-link DI-524 but I have tried 2 other new routers different makes (Philips
& Sweex),

DHCP configured on the WAN (not Static IP address),

Checked settings auto reconnect is ticked. There is something about a Lease
timer in the routers settings that I am not too sure about.

All other setting seem to be alright in the routers config/setup program.

I have tried it with and without routers firewall setting on and off.

It doesnt say 'network cable unplugged' in XP so the connection from PC
Ethernet socket to router must be fine.

It seems to connect when I reset the router and then after a few minutes to
about half hour it will cut out but if I take the Ethernet cable out of the
WAN socket on the router and put it straight into the PC's Ethernet socket
it stays connected (for well over an hour anyway)

Any ideas please?

Andy.