New iMac has trouble with Express - I'm lost on this one

I've just upgraded to a new iMac running Leopard and it's a fine machine.

However, there is one irk.

Out of ever five page I try to browse in Firefox or Safari, I get the 'Try again' dialogue box. If I try again I get the page I want - sometimes - but it makes it very slow.

My broadband connection is fine. I'm running a wireless Airport Express and my iBook never has these problems connecting on the same network.

Someone in the pub said I should erase my DNS server (which is currently set to 10.0.1.1) but I don't seem to able as there is an option to add but not delete this.

As a point of interest my connection says 'AirPort is connected to Apple Network 238f87 and has the IP address 10.0.1.3'.

Does anyone have any clues before I call Apple or call out my local Mac man?

Thanks,

Nigel.