Some game script timings are FPS-dependent, and several missions cannot be finished without lowering FPS. Examples: gtaforums.com/topic/950644-gta-iv-a-revengers-tragedy-heli-bug/ steamcommunity.com/app/12210/discussions/0/864957183136775300/
At least 'Revenger`s Tragedy', 'Out of Commission' and 'Truck Hustle' affected.