What model of PS3 is it? If it's one of the originals then replacing the disc drive with one from ebay by your self isn't hard, just make sure the model numbers match.
Your PS3's spinning mechanism is probably faulty or has just given up on life after so much use. So the disc is spinning slower and the drive is trying to compensate. Eventually, you'll get scratches from the disc being overworked by the laser.
The early (and possibly even the slim versions of the) Xbox 360s commonly ruined it's discs because the drive didn't include a buffer that kept the cycling consistent. So any small movement of the console would permanently etch rings into the discs because the laser would be kept on a certain part much longer than it was designed. I assume something similar is happening to your console
Also first post, long time lurker, all that jazz. Hi!