A likely problem is that you need to lube the laser sled rails, You can search google on more info on how to do this, I had a similar problem and replaced the entire laser assembly itself, however the problem was the sled rails needed to be lubricated. I used some fine viscosity gun oil but thats just because its what i had on hand and didnt want to wait to go get something better.
Theres also some other possible prolems listed on the page here if that doesnt work.
http://www.repairfaq.org/REPAIR/F_cdfaq4.html 
