Thanks for your replies!
Yes, I have to confess I simply installed the x86 version on Win 8.1.1 without really thinking about it (I had been using the x64 version of SE 2013, so I'm not sure what I was thinking... other than that x86 graphics programs have never shown that kind of problem running in the WoW x64 environment).
I've since discovered that the Windows recovery environment on my machine no longer works (reagentc.exe /info returns an error message, even if both sfc.exe and dism.exe complete their checks and give a clean bill of health), so that has to be fixed immediately by an in-place non-destructive Windows refresh installation. At that time I'll also reinstall the Lenovo graphics driver package, as well as the x64 version of RC 1.
Sorry for wasting your time.