OK after more testing I have significant news.
All these bogus messages about GFX cards overheating etc are just that. BOGUS.
I have now discovered a pattern to this particular failure mode of the sim which I hope might lead to Asobo or nVidia or both of them being able to find a cure for this nasty problem.
The fix is simple.
Reinstall your graphics drivers. BUT you’ll have to do this regularly.
In more detail, I installed many different versions of the nVidia drivers over the last few weeks while chasing this problem. I always noticed that the sim works fine for an entire (long) sim session after a fresh driver install. Then I close the sim down for the day, and tomorrow when I start the sim it fails again.
But here is the signigicant bit.
After the sim fails, all I have to do is reinstall the exact same driver as I am currently running, and after that, the sim will work again for just one session.
I have now repeatedly tested this several times.
At this point, all my nVidia GFX driver reinstalls have used the check box for a Clean Install.
Soon I am going to test this without the Clean Install option and see if that works too.
So in summary. Sim works for a session. Sim fails on next session. Reinstall exact same driver I was running on previous session., Sim works again, but for only one session.
So perhaps something, maybe a cache, is getting corrupted either during the successful sim session, or at the start of the very next failing session.
Previously I had been manually cleaning the nVidia DXcache but that didn’t overcome the problem.
Note I do not run a sim rolling cache at all - it is turned off. I’ve tried it with it on and it did not help.
Also I’ve tested with and without a Windows PageFile and this made no difference either on my system.
Maybe others could try the suggestion I’ve made above, reinstalling the current GFX drivers and see if you get a good sim launch next time. Report your findings back here please.