VPN troubles

I’m using a Dell Latitude D810 w/ built-in Wifi, a Netopia 802.11b Wifi router, and Cisco VPN 4.8 on WinXP for my home office. Performance degrades over time from 30 KB/s (DSL) to 3 KB/Min. Reboots help for a bit sometimes. Cisco VPN SW deinstalls/reinstalls as well as VPN profile updates do not alleviate the problem. Using Ethernet instead of Wifi gets the throughput back up (while on Enet, does nothing for Wifi problem). Connecting to the VPN servers takes 20+ seconds when things are slow, and frequently times out (error 427 or 412 usually).

Thanks in advance for any useful tips. I tried Cisco, but they require that I have a support contract I do not have access to.