Please note that windows lies about 'connection speed'. Also the 3/5 bars or whatever is totally meaningless, it's driver/card dependent. Some cards say one thing, others a different thing. Never use this as anything but a guide.
Also as you transmit or receive data the 'speed' will change depending on droped packets, congestion etc. So in reality you'll never get full speed.
Maybe there is something in the way between u and the router? Ie a small amount of packets are ok, but when sent more, lots are dropped etc.
Have a bit play with the location of your router and twiddle the aerials. Who knows, it might improve