Turn up the logging on the vpn clients to high to capture what's happening, if this is to happen again. Its best to see where it's Actually failing, what the reason is that is. Also, you can turn on/up the debug crypto ispec (1-255) to catch what's going on - but the fact that they all get booted at the same time behind the same router.on the clients you can increase the Dead Peer Detection DPD at the bottom of the Transport tab of the client's configuration- default is 90, you can set it higher....up to 480.