Error 789 l2tp vista

Hello folks,
I can't consistently connect over VPN using L2TP, I get error 789. Others using that server can connect consistently. Here are some details.

- I'm using Windows 7 but had the same problem with Vista.
- I can connect sometimes, maybe every 10th reboot.
- I've saved copies of system and application event logs and compared them, there are no differences other than the listed order of items that all happened to start in the same second.
- I've saved copies of services running and they're the same.
- I've tried a clean boot using msconfig with all non-ms services stopped and no startup apps running.
- I've tried removing my home router from the equation and using the modem directly.
- Sometimes it works for a clean boot and sometimes not. Same for a full boot.
- We're using pre-shared key authentication.

Any ideas? I've been trying to get this nailed for two days and it's driving me nuts.