I dug up my old Thinkpad R40 with Intel pro wireless 2100 mini network
adapter and updated it to XP SP3 from XP SP1.

I use a Linksys N router with my 2 other notebook PCs (WAP2 encryption) and
have no problem whatsoever.

I updated the Intel 2100 driver because the old driver didn't have the WAP2
feature. I successfully connected the Thinkpad to the router.

Now, here's the strangest part. The wireless connection stays well-connected
if I simply browse the Internet. However, the Thinkpad loses connection in
20-30 seconds after I start downloading a relatively big file. Then, the
Thinkpad re-establishes wireless connection.

I've tried everything I can think of: disabling AVG, windows firewall,
spy-bot. The Thinkpad had no trouble with wireless connection when I used
WEP in the past. What do you think is happening?