Welcome to the unpredictable world of third party mods. I've had some trouble too with the CSM. Firstly make sure you have the very latest CSM version, it's 1.27c. You can try reinstalling carsets, but I found this often unreliable. Doesn't hurt to try though.
I think the best solution is simply to make a back up of your installed cars. Although it's a hassle, after a corrupt CSM install I've uninstalled GP4 and CSM, done a registry clean, and reinstalled them. Then every time I get a successful CSM install I make a backup of the GPSAVES folder. So if I get a bad subsequent CSM install, I can just go back to the earlier one.