It could be a low spot in the glass. The rings are caused by incomplete contact. This can be cause by an imperfection in the glass, and often just by high humidity. You can try replacing the glass, increasing the pressure on the sandwich (try adding paper under your printing paper) lowering the humidity, changing to the bad type of old non-glare glass, or experimenting with other types of non glare glass (breaks up the harmonic) or blowing the bucks on anti newton glass. Personally I'd start with new glass and work up from there.
Originally Posted by wildbill