> On the Medium course I often encounter something invisible which jolts the car and
> slows it down. It seems to mostly be in a certain part of the track.
I seem to remember ElSemi had an issue with Sega Rally doing similar and fixed it in his emulator, could be those fixes never made it back, or it's a related but slightly different bug.
Virtua Racing is also currently using code taken from Daytona instead of it's own, because there is currently no method to dump the actual internal code on a VR, that could also be causing issues.