I have similar problem with this mission.
Mine freezes after they get on the bike and drive down the ramp. At bottom of ramp just after they go behind wall it freezes. I can alt tab out and back in, 2nd time i do this I get the pause menu (options etc) i have tried to play with the display settings from here and resuming but still no luck.
system PC with p4 3.0GHZ, 2gb Ram(pc3200), 500gb SATA HDD, Asus HD5450 video card
Game version the Golden Penn extreme edition 2011.
Someone suggested to replace the modded bike with the original but I don't know how to do that.
Any help would be greatly appreciated.