OK, I have done a little trial & error & found the culprit. In Options menu, Advanced, it is the High Detail Wheels which is causing the problem, that option needs to be ticked then all works OK.
This rings a bell; seem to recall it being mentioned as required in certain past races, but not sure.
Just finished a lengthy praccy session trying Hristo's setups, regrettably without anyone else joining in. For the heavy car, I was surprised to find his also using the low gearing. I'm running out of revs, and with the presumed higher speeds he would attain, I'd expect it to be even more of a problem for him. Setup gave me problems in the chicane - if I braked into it, the car would almost invariably snap either left or right, causing loss of control - and time. Thus never able to match or better my time with my own setup, which is more forgiving in that situation.
Different story with the light car. Here Hristo's setup allowed me to knock 0.7 sec off my pb, getting down to 1:23.0 - and having tremendous fun while doing it!
Was with a light fuel load though.