My last post on this as it's going very off topic.
I discovered, with the help of you folks and Avon Lady's site (some manic mom, what?!) that it was my video card settings. I'd run a Geforce Tweak utility some time ago, and had actually got things set so OFP didn't like them at all. Took me hours of messing around to get it running smoothly again (it didn't have a 'set defaults' option).
All's well, OFP RES is running in low-ish settings, but runs smoothly. Happy days. ;D