Are you removing the wireless network and re-adding it when you change the network speed? I found that mine stuck at the g speeds even though it claimed it was at n speeds. Delete all wireless profiles and the join the n network.
This is interesting as I am having an almost identical issue. I get 3mb (adsl) with my netgear 934g router but if I plug in an N type router it drops to 1mb. I've tried 5 different routers including the new Sky hub, same thing happens. To test its not the desktop I've plugged in my laptop directly to the the main telephone socket (the desktop is on an extention) and still get the same issue. I have started a thread on this. SKy are scratching their collective heads on this one. Al