a friend of mine had a similar difficulty with an older cd rom drive. don't know if your problem is identical but his simply had dust inside which was just enough to play havoc with the laser. we literally blew the dust away by blowing into the unit and it works fine. if you want to be "techhie" about, you can opt for a can of compressed air. if this doesn't help it's probably mechanical and given the ratio of repair cost to replacement, a new unit may be in order. of course, if your unit is still under warranty, use the warranty.
One good turn... usually gets most of the blanket.
512 Crucial PC2700
WD 80GB SE