I’m now using my LinkSys router again. I’ve flashed it to 1.42.7 (it was previously running 1.42.6 firmware) and turned off the MTU limit in the filter screen, and so far I haven’t lost the connection. I found that the VPN client on my ThinkPad wouldn’t work if I have the Airport Base Station working as a router, but it does work if the base station acts as a bridge, with the BEFSR41 working as a router.