It sounds like your focusing lens needs a touch up on the allignment causing the problem you're having. Usually when the lens needs cleaning the unit will give you an error like "insert disk" or similar to that.
The fact that it takes the disk 3/4 loadings untill it reads it, leads me to beleive the allignment if off, which is a very simple movement of the lens to reallign it. Check with a service company, find out what the charges are. It might be worthed having it done.
Mr.T