PS3 reads some Games other not or badly!

Hi,

my PS3 120 GB have trouble to read Games and to play them.

I thought ok its the laser. But there is one thing i don´t understand.

There are games that runs after some trouble of reading Disc. But then it runs for hours until crash/freeze in the game.

But then some Games it wont read/indentify the Disc.

So I think it is my laser. But why some games are playable for hours and others cant identify.

Oh and sometimes it say´s bad disc data but this is really rare happening.

So what u think? Is it the laser or is their any other thing that can be the problem ?

Thank you for answers.

( sorry for bad english, hope you understand me ) :)