There is a little trick that can be used on a PS2 to correct the DRE (Disc Read Error). NOTE: This -will- void your warranty, and you may permantly damage your PS2 only try it if you are willing to take that risk.
Now for the good stuff:
1)Take the cover off fo your PS2 by removing the screws on the bottom, I believe there are 8 of them. You will have to pry out the little rubber "feet" that hold your PS2 to hard surfaces, there are also 4 more that are not made of rubber. I reccomend using a butter knife for this.
2)Remove the disc tray cover (the black rectangular one) you will need an eyeglass repair screwdriver or something similar for this. Take out all the screws (I think there are 4) and pull up on the cover, it should come right off.
3)Once you have both covers off, press the eject button to get the disc tray out of the way. You should see a little white circular gear. Turn that 1/8 to 1/4 of a turn, what is does is adjust the angle of then lens I believe. This may correct the problem, if not turn it a little more and keep trying.
This is not guarnteed to work, and I clam no responsiblity for any damages to your system using this method. On a lighter note, this was printed in Gamepro and I have personally used it to fix my PS2, so tghere is a good chance of it working