IPSec Site to Site VPN with ERL to ERL not stable even with v1.85

I've got a client with a pair of EdgeRouter Lites and they're having issues with a site-to-site IPSec VPN - the tunnel will drop and then not reestablish itself.

We've been through a fiar bit of troubleshooting, the latest things we have tried are:

1. Upgraded both ends to 1.8.5, deleted and recreated the VPN configuration in the GUI

2. Added dead-peer-detection as per other posts on this forum

dead-peer-detection {
    action restart
    interval 30
    timeout 120
}

3. Set connection-type to initiatie (changed from respond) at both ends

connection-type initiate

The only thing that works when the tunnel drops is to ssh into one of the routers and run sudo ipsec restart

Specifically running restart vpn instead does not work - it seems to send a SIGINT to shut down the VPN, but then it doesn't come back up again.

Any ideas how to make this tunnel stable, and how to make sure it re-establishes itself should it drop out?

Thanks,
Kai

Here's the relevant config from one end of the VPN:

auto-firewall-nat-exclude enable
esp-group FOO0 {
    compression disable
    lifetime 3600
    mode tunnel
    pfs enable
    proposal 1 {
        encryption aes128
        hash sha1
    }
}
ike-group FOO0 {
    dead-peer-detection {
        action restart
        interval 30
        timeout 120
    }
    ikev2-reauth no
    key-exchange ikev1
    lifetime 28800
    proposal 1 {
        dh-group 14
        encryption aes128
        hash sha1
    }
}
ipsec-interfaces {
    interface pppoe0
}
nat-networks {
    allowed-network 0.0.0.0/0 {
    }
}
nat-traversal enable
site-to-site {
    peer 1.2.3.4 {
        authentication {
            mode pre-shared-secret
            pre-shared-secret my-preshared-secret
        }
        connection-type initiate
        description Remote
        ike-group FOO0
        ikev2-reauth inherit
        local-address 5.6.7.8
        tunnel 1 {
            allow-nat-networks disable
            allow-public-networks disable
            esp-group FOO0
            local {
                prefix 192.168.1.0/24
            }
            remote {
                prefix 192.168.2.0/24
            }
        }
    }
}