I just don't know why you ask stuff like this here in the Goldeneye Source forum.
I mean, the chance someone here got TDU and know what's the problem isn't very high.
You should ask in a TDU forum, or a more general gaming community site, or contact the support.
Just a few thoughts:
Get the latest updates for your Anti-Virus and scan your HDD.
Shut down unnecessary programs, clean your system, registry, defrag and optimize.
Try another Game, at best one with high system requirements and maybe you'll find out it's not Game relate, but hardware issues.
Open up your case and clean it from dust.
Check if the fans working correctly.
Get a PC diagnostic tool and check if the RAM and other things are working correctly.
If everything else failed, reinstall Windows.