Intermittent internet access through ADSL router ?

Righty.

Got a problem - a very odd problem.
When I have my ZyXel Prestige 645 ADSL router directly connected to my laptop everything works perfectly.
But when the router's connected to the network, and I'm connected to the network, there's intermittent access.
Now, there are no conflicts of any sort on the network, RIP and DHCP are running on a Win2k box, so those are turned off on the router.

Everything is fine about the router from what I can see, but when its on the network it just ain't happening.
Can't figure it out at all at all. Been on-site for a few days now checking everything I can think of ....
Any thoughts ?

Cheers!
- Jamie.