It happens with almost all cars except for the F-Trucks and V10's.OH btw.....this is happening on all cars and all tracks. Not just the super V8's.
Sorry, I solved this issue by installing the game. please be sure to back up your mods, im in the process of tracking them down and reinstalling them.atm!
I'm not a registered user for that site. And I'm in the same boat as the OP. My full AMS install on steam works great , but my "Stockcar extreme beta " ( which was over written as a AMS beta ) has the issues.
My beta install has all my downloaded tracks and mods installed....so right now it's worthless until they get it fixed. No biggie...as I still have AMS and a version of GSCx V1.51 installed to keep me happy.
Renato Simioni said it's a bug and it'll be corrected in next update.