It could be a dirty lens, or it could be failing optics. It probably wouldn't hurt to try to clean the lens. If it's out of warranty, just pop the top off and inspect. If it looks dusty, wipe it off with a DRY, clean cloth. If that doesn't fix it, it could be something in the laser assy dying.
Fingerprints and scratches can cause this too, but it will happen in the same spot every time if that's the case. I've had it happen with perfectly good discs as well. My old player is around 6 yrs old and it started to do this randomly too, so I replaced it. The old player still works, just doesn't get used as often.