Do the logs on the router show that the circuit did not drop
You could try run ip sla with interesting traffic from the vpn allowed subnets to keep the vpn up but if the line is failing the vpn will fail
The fact the vpn is coming back by itself suggests the config is good so you need to find whats causing it to fail ,usually a dropped line is the cause but you can check the logging in the router and run some debugs to try and capture whats happening
vpn will not come backup after a failure until interesting traffic hits the router thats part of the vpn subnets