IPSEC DOWN UNTIL REBOOT

Have you done any troubleshooting during the down time?

Try doing a packet capture of connection attempts across the tunnel when it isn't flowing traffic. See what happens there. That should give you a pretty good start as to where to look for more information.

The hint for using blackhole routes is valuable. Search the forums for 'blackhole', you should find my ready-to-use script in insert bh routes for all private address ranges.

The point here is: these bh routes have the highest priority, that is, the highest cost. So they won't be used if there is a better route, the one across the VPN tunnel. When the tunnel goes down, it's route is deleted and all traffic follows the bh route into Nirvana. Thus, no sessions are opened and the VPN can be re-negotiated immediately.

The real underlying problem with your VPN is that the SA expires without prior re-negotiation. Look into the  'auto-rekey' and 'auto-negotiate' options in the docs and then in your config. You should also set up Dead Gateway Detection, i.e. a ping server, to inform the FGT immediately when the tunnel goes down.