SafeCom SWMULZ-5400

I'm trying to use a SafeCom SWMULZ-5400 adapter with my Linksys WRT54GS and
WUSB54GS.
I want to use it to talk to a windows 98SE PC, but can't get it to connect.
I tried it on an XP machine, and it connected once, but hasn't worked on
that machine since.
It appears to be tying to connect all the time but rarely succeeds except
for an occasional few seconds.
Signal strength and quality are excellent (on the one occasion it worked,
but are also excellent on the Linksys devices).
Some advice would be appreciated
Have I got a duff adapter?
Are Linksys and Safecom incompatible?
Any suggestions

Cheers,
John.