Problem with Router or PC?

I have a Linksys WRT330N wireless router and a Dell XP Pro laptop.
Normally I connect with via cable, but a couple of days ago I
was running wireless (Intel Pro/Wireless 3945ABG) and it works
for a while but then it degrades to where this is no data transfer
and I get timeouts with browser and email. I have had this setup
for about 5 months and haven't had any problems.

The status on my PC indicates that I have a strong signal and
running the Intel wireless diagnostic of my PC shows no problem.
When I connect back to the cat 5 cable, it works fine.

Any idea on how I can determine if it's a router problem or
the card in my PC (or another problem)?

TIA