The racers flickering in Winter Gold is due to timing. It's extremely sensitive, and very tiny changes often break the graphics again on me.
EDIT: finished checking all of MESS' opcodes against mine, I don't see any errors. Ah well, either I'm overlooking them as well or they lie in the S-CPU core.
I dunno, I'd say both have similar likelihoods. I'd like to say it's the S-CPU core, but I know perfectly well that I visually scoured my SuperFX implementation 10+ times before I discovered my HIB typo.