Thank you for the answers.
I found a monitor function in Diagbox, and the cell voltages on CMU12 looked like this
As the dropouts are completely random it looks like the IC that reads the cell voltage is failing.
The IC is from Analog Devices and I just happened to have an account there, so I ordered a couple as samples (FREE
But can it perhaps be the power supply section that is causing this?
I guess that it is a lot of work to remove and replace the battery (?) just to test this, so it would be better to be on the safe side.