Dvd's that do not work well!

[BigBoss]

Hello, I have a v5 ps2 modded with messiah. I have been playing my dvd-r's without any problem for a lot of time, but recently, they do not work very well. For example, when I'm playing gta vice city, suddently the game freezes and stops loading. but this happens also with mgs and ffX! can anybody tell me why my ps2 does not load my games correctly if at the beginning it did? can it be solved (perhaps it is a lens power problem?)?
Please help me.
Thanks a lot!