i used to get this after adding car models to cars at save locations. I would be fine until I drove the new imported model to/ near the save point, or try to drive it away from the save point. What you can do
1. delete the save and start again OR
2. find out what car(s) are causing the error, replace the car with the original car (the car you drove to the save point and saved there (this may not be the GTA4 original)), load up the save and move the car out of save point, save again. You can now Import the car again and hopefully save it without any problems. From my experience it seems YOU CANNOT MOD A CAR IF IT'S AT A SAVE POINT!
I don't bother saving cars anymore and have no more problems ;-)
for video 'my smpa60 explained and solved gta4'- http://www.youtube.com/watch?v=r9snCs4z2K4