Internet connection dropping and reconnecting periodically

We have a two year old Dell Inspiron 6400 complete with Intel PRO/Wireless 3945 Dual Band 802.11a/g 54Mbps Wireless Mini Card and Windows XP.

For the last 6 months or so we have had our internet connection drop and pick up periodically every few minutes.  Some days we don't have a problem and then other days we have a constant battle staying connected.

We have had a PC technician come look at it and ran something on the prompt to see whether the wireless card stays connected - it did for the hour he was here but didn't try anything else but swap our Bitdefender security to a 30 day trial of Norton 360.  This didn't seem to make a difference so the trial for 360 ran out and reinstalled Bitdefender since we paid for another 100 days.

We have a Belkin N router but don't think it's the router or modem as my husband has a Dell laptop for work and if we connect at the same time, his laptop behaves and stays connected.

I'm at a lost on why it keeps dropping connection and whether to spend money to replace the wireless card.

Can anyone help out there??

Cheers TeamSmith