Forum:GTAIV problem: graphics lag prevets mission completion

hey guys

I cant complete 'A revenger's tragedy' since every time I have to climb on the chopper I fall down

Each time I press space the game freezes and the game doesn't register it...

therefore I fall down about 1 minute after I manage to get beneath the chopper

the same problem occured when played the mission 'Truck Huslte'

please help me!