you can try a readily available cd lens cleaner... its basically a CD with a small brush on it to clean the lens.

Being dirty wont really cause an intermitent problem though, if its dirty it would skip a lot and the problem would be consistent, not just once in a while. It sounds like the cd motor is bad in yours as well... If you want me to take a look at it, pull it out and send it to me... i can fix it.