cpu usage problem

[simonchen]

I am using T40, with 1.3G cpu. I am using win xp.
My original wireless card is intel 11b, and I upgraded it to IBM b/g.
I am using a D-Link DI-524 router at home. The problem is, when my laptop is near the router, the cpu usage is fine. But when I move my laptop a little far away, say upstairs, the cpu usage will immediately grow up to 100% and everything becomes slow. Same thing happened when I was using a buffalo router.
Interestingly, when I turn my screen off, the cpu usage will drop.

Anybody has met this problem before? Really need your help!
Thanks