XGS 3100 becoming unresponsive and requiring hard restart

We have an XGS 3100 which approx. every 1-2 weeks becomes unresponsive and requires a hard restart as it completely stops functioning. We have a smart plug connected now, which automatically powers it off and on again if the network goes down to mitigate the issue.

How can I find the cause of the issue? We've had a replacement device from Sophos but with the same config the issue persists.

Our network is on 255.255.254.0

We have configured IPSEC and SSL VPNs

We use a Horizon VOIP system which required us to increase the UDP NAT timeout to 600s, and disable SIP.ALG

We have a web/ftp server in the DMZ

All I can think to do is to reconfigure it myself from scratch on a weekend, as I don't know what other admins may have previously done with it. But I'd really prefer not to.