USR Wireless & Linksys 54g Router

Hi,

I am having major problems getting a US Robotics Wireless Turbo PCI card
working with my Linksys WRT54G router. Basically it works if I grab hold of
the aerial at the back, but as soon as i let go, things stop working again.
I know it is not a problem with the card itself, because I have tried an
identical card in the same pc without it working and that card has worked in
another pc with no problems. I am really puzzled about this.

The PC is newly built - Abit KV7 motherboard, Athlon XP, GeForce 4, Windows
2000 SP4.
I have put the latest firmware on the router.
Also, it appears that the signal strength indicator on the monitor app is
updated continously, even though no data gets through unless i hold it.

Any ideas?
Thanks in advance