I'll take a guess here, because you said it's happening to all your cars. Are the cars maxing out, but not necessarily going very fast? If so, I'd guess that you were using the automatic transmission before, and that setting has changed to manual transmission. At the start of a race, you don't do anything to change gears because you think it's an automatic. The cars reach max revs in first gear, and it bounces off the rev limiter, waiting for you to change gears.
So if you're seeing this at the start of races, then I'd check the manual/auto tranny settings.