I'm trying to connect to a L2TP IPSec VPN from my home using the Win 7 x64 VPN client. Sometimes on a fresh reboot, the connection is successful. If I then disconnect and try to reconnect, the connection fails. Sometimes I can't connect even from a fresh boot. The computer at home is behind a wireless router. There is no problem doing the same from a WinXP VPN client on the same local network. Any suggestions?

I'll add that it makes no difference if I turn off all the firewalls.