Random Lag spikes and disconnections

[British_Command]

I have a D-link DIR-635 router and a wireless N adapter for my Xbox 360 console, there are about, in total, 8/9 devices connecting to this router of which 2 will be playing online games (Xbox 360 and PC), and about 2/3 laptops are on the internet, the rest of the devices are not in use. My ISP is Virgin Media and 10MB connection, the issue I'm having is that I seem to randomly lag on my Xbox 360 but my brothers PC games work fine, as well my 360, my laptop (located in the same place and both connecting wirelessly) will disconnect from the internet and I'll have to turn the router on and off to resolve the problem yet my brothers computer will not get disconnected (it's closer to the router). I do have a wireless phone base quite near to my router but I have only started experiencing this problem for the last few months and yet it was fine before and I had changed nothing to the overall setup.

I've tried changing channels on my wireless router and it already has a function to find the optimal channel, I've put my 360 in a DMZ and this has made no difference to the random lag.

Any other ideas?
Thanks Guys!