Seems like a fair amount of peolpe don't like this card.
One person reported similar problems as you did...
http://att.com.com/Linksys_WMP54G_Wi...=top&fb=0&ob=0 ...his soluiton was to get a different card.
Linksys support site doesn't show any obvious problems like yours.
I would focus on freeing up (& assigning) an IRQ manually to see if that helps.