Since my last post I've had my server running on my bench, where I moved it to perform the memory test on Sunday, 4 days ago.
It normally lives in an out of the way place on a UPS and with only a network connection.
On my bench it is not using the UPS and has a keyboard, mouse, monitor and network plugged in. It is also started in GUI mode.
It has now been up for 3 days 10 hours 22 minutes, which it has not been doing for months.
Could there be anything involving the UPS that might make it crash & restart without anything useful being logged?