DNS Errors

On Thursday I bought a new router(sumvision 802.11b/g wireless broadband
router). I already had a router with four PCs connected to it and all seemed
fine, I moved one of the PCs and couldn't be bothered running a network
cable, hence the new router.

I have it set up as follows,

3 pcs conected to the ethernet ports on the back of the router, 1 pc
connected wireless.

Router set up to be DHCP server on the LAN side, DHCP client on the server
side and the cloned MAC address is the same as in the old router.

2 pc connected to the ethernet port run winxp the other, redhat linux and
the wireless pc also runs winxp.

4Mb Cable connection(telewest)

The problem I have is that when I set things up, everything worked as
expected but after a while, web browsing started to slow down. I turned the
router off for a while and then powered back up again and my browsing speed
was back to normal. I left the PC for a couple of hours and on return, web
browsing had again slowed down until eventually I got DNS Errors every time.
Every PC was the same when browsing.

I noticed that the performance of online gaming was not affected in any way,
either me accessing another server or anybody accessing my own. Similarly
when the browsing was slowing down, I downloaded a file at a rate of
525Kb/s, which is about average for me so it only seems to be when browsing
that I have this problem.

I disabled the wireless settings in the router, to take that out of the
equation but the same keeps happening, ie browsing slows until eventually I
get DNS errors, I reboot router and it works again for a couple of hours.

When I do ipconfig, everything is as expected.

Is this likely to be a hardware fault or something I have done wrong.

Any help would be appreciated.