This started happening after I added a 'Bridge' broadband connection which has worked but is normally set as inactive as I use a VDSL connection.
Could you clarify what you mean by this? If you are using the modem in bridge mode, i.e with another router handling the connection with just the VLAN stuff set in the zyxel, then you would need this 'bridge' connection in the 'Broadband' section set to active. If you are not using it like this then there is no need for adding a bridge connection in that section at all.
If you are using it in bridge mode, how are you accessing the modem? A second ethernet cable going to your LAN side of the router? Have you set up the interface groups to make sure that works on as per:
https://kitz.co.uk/routers/zyxel_VMG8324-B10A_bridge.htmOther than that I'd say the first course of action would be a fully reset, maybe flash the ver 24 firmware and also ensure TR069 remote updates are disabled, its unlikely that your ISP would push an update to this model, but best to be sure.
If you have the patience though and a suitable replacement may I suggest trying a different power supply? I know it sounds unlikely, but given no changes (other that the bridge connection thing) on the software side, and the intermittent nature of the fault, it could be a flaky PSU?