Tenda pci adaptor +bebox = bad internet??

[silverback]

Just bought a pc and it came fitted with a TENDA W311P 150Mbps wireless pci adaptor fitted, now the problem i am having is while my mobile phone and my dell laptop can pickup the signal from my bebox router(downstairs) from my bedroom, the tenda also can,BUT it seems to be loosing the signal all the time and the internet goes quick 1 minute then stalls/stops for ages to the point where it wont even load google? Yet in the setup page of tenda it says my connection to router is 70-80%. Now when i brought the pc downstairs so its vertually next to the router as expected the internet is fast as it should be. Can not figure it out?
Any idea's for a complete non computer savy newby.

Thanks