As CrystalChip said it may be your laser, if it's a V12 model without an adequate laser fix, that's a likely possibility.
From my experience with my slim, it has been extremely media fussy since day one (many of the highly recommended brands here don't work too well on it, most Jap media is fine however) so it still could be media. If you can find it, try and get your hand on a few Japanese makes of media. (look on the packages for "Made In Japan") and see how they go. It also may be worth trying some +r media if you have easy access to a decent brand.
Failing that, it might be time to change out the laser, which is a fairly quick and reasonably inexpensive process on the slimline consoles.
PSTwo V14 SCPH-75002a: Crystal Chip 1.1
XBox 1.6b: SmartXX v2.0