Tonksi, ive fixed many a "dead" cd or DVD by taking them to bits and cleaning the heads and removing the pushin ribbon connectors and reseating them. The prob u have got sounds like its a head problem but if you say it works on some and not on others i dont fair that cleaning the head will have much hope, its sounds to me like the laser maybe on its way out.
The easiest way to open a cd or dvd is to first open the draw when the drive is powered and then shut down the pc with the draw open. Remove all screws from both sides (2 each side) and then unclip the front that draw seats too ( u can only do this with the draw open).
Romove the top and the bottom plate and then u will have access to the ribbon connectors and the laser head its self.
Clean the head with tissue paper (dry) and remove the ribbons and reseat them back into there housing (after cleaning the contacts)
Reseat everything in the reverse order and then try your luck.
PS Please be careful not to dislodge the cd draw ( pull it out slowely)as its a pain to get everything back in line if it slips on a few notches on the rail etc ( youll see what i mean)
