This year my relatively new GMC-300 geiger counter started having spells of measuring background as 300-2000 CPM. I took it and drove miles around, and still had same unexplained elevation. Yet no one else was reporting elevated readings. Finally sent it back to manufacturer and after a couple of weeks, they sent be a new one--which reads 15-40 CPM as it should. I also tested both with a known source, and both responded with increased counts.