What does popping mean?
Anyway I gave you all the knowladge I had. My PC crash with vehicle density higher than 20 so I can't help you. And I had to uninstall GTA IV coz somehow when I put skin via Open IV, evrything has been deleted LOL Don't play with player skin - risky.
When you downlaod a car there is a readme file. There are HANGLINGS settings for each car. You must copy the whole line with numbers and with a name (turismo ect.) and replace the original handlings settings located in gtaIV/common/data/handling.dat....you must also change numbers in the vehicle.dat also located in the same place.
Sometimes Vehicle Mod Installer doesn't see HANDLINGS hidden in the readme file properly and you have to do that manualy.