I always clear caches when starting a game, by using a batch file. Research on the internet suggested it might be the graphics card overheating but it still happened after the computer had been switched off overnight. Starting a new game in a completely new folder, playing bog-standard Sunset Valley, was worse if anything.
System Restore to a point a week ago didn't help.
It could be a graphics card compatibility issue (ATI Radeon HD 5600 series, which does seem to have problems with Sims) - but it's been fine until now and I can't even find the option for the fix suggested on the Sims website.
I really don't want to do another complete reinstall. With my slow internet connection, it takes a whole day.
Off to try uninstalling and reinstalling drivers - and then maybe to look at the price of graphics cards