Tjyven - as a fellow Mara X user, I follow this with interest and will wait to hear what the experts say and hope you will find the cause and a solution.
From what you describe, turning the machine off and on again fixes the problem? If so, wouldn’t that point in the direction of a software issue (so maybe the Gicar box)?
As for Dave’s question about the water, I think the point isn’t if the water tastes good, but rather if it will scale in the machine. My tap water tastes very good to me, but it is also very hard, so can cause problems if used in an espresso machine. You should know the scaling potential of the water you use in the machine, and if it is going to scale - better do something about it to prevent problems rather than try to fix them later. For example, if your temperature probes are covered in scale, this can affect their ability to read correctly and could cause temperature issues. However, if a simple on and off cycle fixes the issue, common sense would be that the problem is somewhere else?
Finally, do you actually need the machine to be on for so many hours? Do you use it all that time? I can’t imagine the constant heat being great for the longevity of the components. Perhaps a smart socket and a programmed schedule will enable you to have the machine on for shorter periods and avoid the issue?