Hi Kaos101 - as a suggestion, can you try changing the AP channel on the router? i've had a similar issue before (running a near identical setup as yourself), which seemed to only affect the imac, which was near a front window of the house - a close neighbour was broadcasting on the same wireless channel, which didn't affect the Win laptops or anything else that was 'mobile' - by a quick process of elimination, i found out what channel he was using (i'm guessng your on a/b/g - 2.4GHz?) and selected a different one and it solved the issue for me....
If you've already been through these hoops - check your logs and make sure that it isn't a DNS issue. In which case, and as a 'belts and braces' approach - try assigning a static IP and see what the result is?
Hope this helps - and the weather better be good up there, as i'm 'Chistmas'ing' in Loughborough!