It's called bottleneck effect for nr2003. It's been told about 100+ times there already what to do or how to fix it. Look for topics.
As a hint I can give you that option - try to compress car files with WinMip2. Other one option- tweak graphic settings with this http://bigdonlinemotorsports.com/NRgtweak.php. If you have good (really good) HiEnd PC you can try to do this way -