My LAN speed is usually around 5100 via my SpeedTouch router ST585v6. When I am using my Laptop, an HP Pavilion dv6 running Windows 7 with an AMD quad Core cpu, not the fastest of computers!, I seem to get WIFi speeds as low as 1700 whilst at other times can get 5000.
The Router is only about 6 feet away from the Laptop! Is the speed drop a "computer problem" or a router one? If I connect the Laptop to the LAN the speed will always be at about 5100. My desktop computer is connected to the LAN and has a speed of 5100.
I have the usual security set on the Router.
Does anyone have any thoughts?
Almost certainly interference on the wireless channel. If there's a practical way of having the ethernet cable attached at all times you will have a far more stable connection than you ever will with wireless. Short of that, try changing the wireless channel that you're on.
There's a free program you can download called inSSIDer (download here) which will show you which wireless channels are already in use by other Wifi networks, but be aware that other wireless devices (such as cordless phones) can cause interference too, and these will not show on inSSIDer.