Try cleaning your laser and also check the laser cable - it may be worn/damaged - check out [http://nikosapi.org/wiki/index.php/Replacing_The_Playstation_2_%28slim%29%27s_Laser|THIS GUIDE]
[http://www.lightinthebox.com/Replacement-Laser-Ribbon-Cable-for-PS2-Slim--SCPH-7000x-_p156654.html|THIS] is the cable
Also have read of [http://fatedtoend.com/content/slim-sony-playstation-ps2-disc-read-error-fixed|THIS ARTICLE]