I've experienced the exact same problem on the exact same hardware across multiple firmware revisions with a different client system and OS to you. WLAN access through the 'net disappears, or performs unbelievably slow, or the WLAN drops out and reconnects intermittently. One other symptom was being almost unable to reach the admin interface for the device via the WLAN.
I tried disabling the intrusion detection and firewall elements of the router (via telnet), and messing with the wireless channels, and QoS... nothing fixed it.
In the end I swapped the unit out for a Linksys WAG200G and have had approximately zero problems since.
I personally believe the model is junk, and there's little to do to remedy it's issues.
If you're in NZ and on Telstra the only thing to beware of when setting up a replacement is that VPI/VCI settings need to be manually set at 0/100. Username and password can be anything you like, it all works.