When the laser starts going bad it will be inconsistent reading games. Might read em one time then not the next time. Some games are harder on the laser than others.
There's only a few things that can go wrong with a dvd drive
- the laser
- the spindle motor that spins the disc
- the worm motor that moves the laser
- the rubber belt that goes on the pulley that opens the drive door
So you basically have to play mechanic. If you're getting a bunch of grinding noises a lot of times it's the spindle motor. If it's having trouble reading the disc and no grinding noises it's either the laser or the worm motor.
If you had another drive of the same brand around you could experiment and swap out parts and see. But if I was to bet I'd go with the laser. Of course for $20 or so you could get a new laser and worm motor. Or see if your friend will let you experiment with parts from his drive. Good luck!