I have (am having) a similar problem with my Linksys Wireless PCI card and WRT54G wireless router.
In addition, on losing the connection the PC is usually unable to connct to the router afterwards - it sees the SSID, but fails to register, saying it needs the network key (it is WEP enabled).
I have the correct key and if I keep refreshing it eventually connects.
This was driving me insane, especialy as the laptop and PDA on the same network have no such problems.
I rang Linksys tech support and they suggested I change the channel (but only through a wired connection in case it disconnects during the update).
The channel is changed through the setup menu at the usual IP address.
Frankly, I was sceptical, but agreed and moved from channel 11 to channel 5 (purely random selection).
It has made an enormous difference. The signal is better and far less volatile. Altough the problem still emerges from time to time.
Give it a try (if you haven't already) and let me know how it goes...