If you're getting the occasional stuttering in the racing it's becuase of the replay data being wrote to the system drive, try the below fix it clears up the stuttering for me.
3# Frame Skips, Low FPS, Lag
PC version of the game may be experiencing all this due to a massive replay file, too much data writing occurs on that file causing the lag and frame skips, when you go back to the game after having watched a replay.
You can find the file here.
1. Delete original replay folder (and file) by exploring to
ProgramData\Codemasters\DiRT3\DataCache\yourcomputersname\
2. Open Command Prompt with admin privileges (search ‘cmd’ and right click on cmd.exe and select ‘Run as Administrator’)
3. Use mklink to create the symbolic link. Edit it according to your own game directory location.
mklink /D C:\ProgramData\Codemasters\DiRT3\DataCache\yo urcomputersname\ E:\Dirt3\replay
4. When launching the game, the 2GB replay.pbf file will be written to the file location you have specified. If you write this file to another hard drive, resources are freed up for your OS and game performance will increase.