The problem turned out to be nominal dust and incomplete contact of the plastic pushbutton stem against the micro switch actuator head. After cleaning the inside of the mice, I spliced a carefully cut small piece of duct tape to the end of pushbutton stems. The pushbutton stems now make firm contact on the micro switch actuator head. The mice are acting like new. No missed clicks, no double clicks instead of single cicks, no dropped select-and-drag problems. Darrell