I've helped someone else with a similar problem here: http://www.fixya.com/support/t1300166-playstation_2_console_keeps_freezing
It's not enough to use a disk to clean the laser lens. You have to take the PS2 apart and clean the lens manually. Nicotine, Incense, diffusers, and other smokes and just time itself clouds the lens. Not only that but the part that the disk sits and spins on needs to be cleaned over time as well to assure the disk is in proper alignment and Is not just kinda spinning on the spindle.
In that link i posted above you be led to directions on how to take apart your PS2 and what parts to clean. The instructions I have posted in that tutorial is for the (FAT) PS2 model not SLIMLINE. If you have a slimline model you don't have to take apart anything just open your cover and clean the parts in the video.
Hope that helps,