Shutdown and remove the DVD drive. Remove the 4 Phillips head screws from the bottom and remove the bottom plate. Use a large paperclip to push into the small hole in the front to release the tray, and extend the tray all the way out.
Now take a flat screw driver to push in the three tabs that hold the face plate, and remove ( the tray will hold it ).
Remove the top cover. Inspect the inside. Take a can of air and gently (from a distance) blow out and dust, etc. Be careful not to damage the laser lens.
Look at the rails and worm screw that control the laser position. Clean these. I used WD-40 on a cotton cloth.
Time to lube. Silicone spray lube is good, yet my favorite is Dupont Teflon Multi-Use Lubricant. If using this follow instructions to shake well and let the Teflon dry after application.
You want to lube both rails and the worm screw.
Gently, yet firmly, move the laser sled back and forth the full extent of the rails until it moves very easily.
Now you gently clean the laser lens (90% isopropyl will do) and reassemble the DVD. Re-install.