Jump onto Jacobs helicopter rails.. Tap like crazy.. & fall to your death. Sound familiar? This is how to fix it..
The problem is caused by the program not being able to handle modern machines & Windows 7. (The frame rate is too high.) To fix this all you need to do is download & install a free video capture program called FRAPS. Once installed, Start FRAPS, Start GTA IV & play the mission. (FRAPS will show your frame rate in the upper left hand screen. Usually between 50 to 80 fps.. Too high.) When you grab hold of Jacobs helicopter press F9 to start the 30 second video capture. (This will slow your frame rate to 30 fps.) Tap like crazy & watch Niko finally climb into the helicopter..
You also get a 30 second movie of Niko not falling to his death saved to you computer for you to watch at your leisure, Email to friends, or to just delete. Anyway the point is you are now in the helicopter & completely free to finish the mission.. Or to slam into a building & fall to your death.