BT Infinity Wireless Probelems

I am an IT Engineer, working in the UK. Recently BT have rolled out thier 'Infinity' service http://www.bt.com/btinfinity for ultra fast broadband.

Now I have lots of customers calling me stating that they have wireless problems.

Upon further inspection, any computers plugged directly into the new HH3 router (HomeHub3.0) are getting super fast speeds of between 30-40Mbps. Problems occur with wireless machines, even one plugged into an 85Mbps homeplug kit, which previously worked fine!

The problem si that on initial startup and speedtest, the wireless machines are reporting 25Mbps n average, which is great. Give it half an hour, and a few restarts, and the machines are getting anything fro m0.5Mbps to 1Mbps.

I have tried changing the channels, encryption, updating the routers FW (it's up to date), I have even tried new 'N' cards/dongles in computers (desktop machines) and tried to connect via new Laptops with the same results.

Even my iPhone 4 will get a dodgy connection. This leads me to thinking that the router is at fault. I have read a thread online where someone has replaced their router, with no improvement on results.

Any ideas anyone? Anyone experiencing similar issues?

many thanks!