Had an evening to experiment and things are starting to play up now. One moment I can be getting a good speed and then it'll suddenly freeze for about 10 seconds, then I'll get full speed again. The router doesn't loose sync and during the periods of no data I can still load up the router config page
I've narrowed the cause of the problem to my plasma tv. Whenever it's turned on all throughput stops although the router stays synced. I've tried relocating the router in the house to no effect. Is it possible to go back to the old service since that worked fine?