That sounds very much like disc rot. It was a lot more common problem with laserdiscs. On DVD and blu ray it seemed to be almost completely eradicated but it does still happen sometimes. As far as I remember, no one
really knows for certain what causes it (though there are some credible theories), but it seems the tighter the tolerances and higher the "clean room" standards at the factory, the less likely disc rot is to appear. Thus, laserdiscs pressed in Japan were usually considered the safest long term to avoid disc rot.
Anyway, take a VERY careful look at the readable side under really good light. If you see any tiny dots or little circles that are on the shiny surface, but INSIDE or under the outer plastic coating, it's disc rot. Also, if you hold the disc up to the light and can see little pin pricks of light shining through... it's rot. In its early stages it can be very tough to see but it can still make a disc nearly unplayable. As it progresses, it WILL render the disc completely unusable.