I've had probs with Titan Quest from day one. Getting poor performance even in low resolution with medium detail settings, mentioned some of the problems encountered in the First 15.

Thankfully a patch has been released bringing the game up to version 1.08. There are multiplayer fixes amongst other things but the main one is fixing the memory leak so should sort out some of the problems I encountered with performance.

Gamershell have the patch mirrored locally here :

http://www.gamershell.com/news/31287.html