Found the issue. Finally managed to capture a crash with a specific event log. Whenever the whole pc crashed of course, all I would get in Event Viewer was the "PC reset unexpectedly" whereas yesterday, tinkering with unloading apps from my system tray I finally managed to get a CTD instead of a full hard reboot.
Kernel.dll was the faulting module. Instantly one thinks of ROM and RAM.
I unloaded everything from my system tray, reverted the startup application set to the bare minimum, loaded up the game and I can report all 4 training missions ran fine with no crashes.
Now to try modding up. But I think we've found the culprit, Meiner Herren!
Thanks Propbeanie, for all your help. Hopefully my solution will also help some Commanders who may have had the same issue. Can't believe it comes back to that old chestnut of memory limits.
Huzzah!