Hello Rob (@bioscythe),
Have you got intouch with your local Peplink Partner as all Peplink Partners have a certified Peplink Engineer onboard and they may be able to help you isolate what is the cause to get it resolved.
Have you got your Balance One setup within InControl2 ?
InControl2 is a great tool for monitoring and in helping to diagnose issues. InControl2 could also help you (as an interium) to setup a reboot schedule (say once every three days in the evening) to do the reboot for you.
Also could you quote your Peplink Support ticket number here for the Peplink team so they can reference back to your ticket for you.
Happy to Help,
Marcus