I have a millermatic 250x mig welder I purchased several years back, new. Having problems with the output voltage suddenly raising very high while welding.. normally have it set around 19-20v but spikes really high causing a huge arc.. I voltmeter tested the output of +26v with load (arc) and higher without a load. Any idea what might be the problem? One minute it will work fine and the next it acts stupid.. any help will be appreciated..

John