Bugger - thanks PC, but one of the applications is a brand new copy of Microsoft Flightsim X Ultimate (I wanted to learn about the Garmin 1000 fit).
This starts all the menu pages perfectly, but on clicking "Go" starts to rythmically flash from screen picture to black and back again every second, very much like a metronome.
Have a couple of hardware bits that don't have 64 bit drivers.