1) check if the game runs on single core affinity (it should by default after installation but it's better to check anyway)
-- to check: start the game, run Task Manager (press Crtl+Alt+Del), switch to 'Processes' tab, find 'thief.exe' and right-click on it, pick 'Set Affinity...' - only one core should be used

2) depending on your GPU, try forcing VSync through NV Control Panel / Catalyst Control Center