Denon DVD-9000 not reading

I have a Denon DVD-9000 player that doesn't always read the disk. Sometimes it locks up, skips, or says the disk is not readable. It doesn't matter what the age of the disk is. It won't read the newest one, but will read some copies. A cleaner was used, then the machine was opened to check for dirt, etc. I sent the unit in for repair and a new optical sensor was installed. Less than two months later it started doing the same thing, and another optical sensor was installed (under the 90 day service warranty). Less than three months later it is now doing the same thing. We don't use this DVD player that much, either.

There is something very defective in this high-end player, and I would really appreciate finding a lasting solution.