Specks of dust in just the right place in the optics could cause this. Pull out the CD-R drive, take the cover off, and dust it out with canned "air." Use short, careful, low-power air bursts around the laser eye area, as the focusing mechanism is fragile. Assuming it's not bad media (easy to rule out) or a PSU problem (more difficult to rule out, but if you're not experiencing other stability issues, don't jump to conclusions), this is the average user's best hope for fixing a malfunctioning optical drive.