This is a serious known problem with PS2's typically some of the early generation have incorrectly adjusted lazers, and with time they become weaker, and start to totally fail.
Here is a great do it yourself guide, thanks to the fine folks at Ars technica:
http://arstechnica.com/articles/paedia/hardware/ps2.ars
I have a release-day PS2, and I have some problems on blue disks, but everything else is okay. (One failure in 50 booting from regular DVD's)