I seem to be fixing a few multimeters recently. This time an Extech EX330. It gave erratic voltage readings. I found that the Volts input jack barrel was loose in its connection to the strap that linked it to the board. These parts are riveted together. I tightened the connection by hammering a punch into the rivet. I also cleaned the rotary switch while I was there, but it was not bad so almost certain the problem with the socket connection.