Linksys WMP54G - weak wireless signal

I have the Linksys WMP54G PCI card and the WAG54G wireless router. I
get excellent signal and speed (54Mbps) when the two computers are in
the same room (no suprise there). However, when I move the WMP54G
computer to the next room, the signal and speed drops (speed down to
12-20Mbps). If I move the computer up or down a floor in the house,
the speed drops down to 1Mbps!!!

Anyone know what is causing this?? Or have similar problems? What is
the expected speed for Linksys hardware when PCs are on different
floors of a house? It's a bog standard house.

Is this a Linksys problem only? Any other hardware users reccomend
their gear? BTW - I am in the UK.