Maybe this could help you:
Q: Cars vanish from the track, get invisible and appear back on some track-spots. This is happening on all tracks., or I can't see all cars at the race-start
A: Please check if the helmet files in the ..gp3jamsh\liveries\rh2002\helmets-directory and the helmet files in the gp3jams\liveries\rh2002\helmets-directory are write-protected. If they are not, please enable write-protection by right clicking on the filename and choosing "properties" from the pop-up-menu. ALL helmets files MUST be write-protected.
Happy racing from germany !!!