There are many reasons for this and I've learned to try other channels and routers. You'll find that such is not unusual and like horse races the winners will swap positions.
Yesterday I got together with some friends and we all hook up to the wireless network in the house to work. Something interesting came up, the Macs couldn't be more than 15 feet away from the router (Netgear router) because after two walls the signal was gone. However the Sony PC (using a D-Link) and another Dell PC didn't seem to have that problem, after 30 feet away and a dyrwall and a cynder block wall the signal only went down to 88%. We were all shock because Macs are famous for quality but the wireless sucks? What are the chances of having both MacPro do the same thing and two PCs doing the same thing. Could we be doing something wrong?