mines was doing the same thing i took the DVD drive apart in the DVD ROM they are to silver poles witch the laser rides on and the have to be greased or the laser will become sluggish and give you trouble well there was some dust in the pole and must haved sucked up all the grease so i took them out greased them put it all back together and it played gta4 right of the bat then about a week ago i hit the 360 while i was backing away fROM putting gta4 in and blam scratch scratch and crack i taught i broke gta4 buuuuuuuttttt i broke the laser and **** the hell out of my gta4 so i got the game replaced by Microsoft and laser from llama.com and all is good

gta4 + xbox360 = happiness