First ensure your discs have no fingerprints or smudges, it will often cope with scratches but rarely fingerprints.
The symptoms are often due to faulty sled motor or contamination to sled slide or sled worm gear. They can be cleaned but it's really a job for an engineer as some disassembly may be required, which would require manufacturers disassembly info. or you'll end up in a knot.
Otherwise, suspect a dirty or faulty laser pick up.
A dirty laser lens can be cleaned but use extremely light pressure as it is on a fine suspension. Simply breathe on it and use a soft cotton bud if you do not have cleaning fluid.
A faulty laser pick up is usually uneconomical to change due to low cost of new players.
There are other possible causes but the above are the most common.
Cobweb
May 2005