Likely a short that is causing the tube to intermittently arc. You might try taking it out if its socket out when cold, and while holding it upright, tapping it sharply on the side a few times with a chopstick. Sometimes the short is caused by small debris from the manufacturing process that gets caught in the filaments or plates during the tumbling of shipping, and, sometimes, one can jostle it out to the bottom of the tube where it will remain out of trouble. About 50% of the time I've been able to resuscitate a faulty, sputerring tube wth this method. Power tubes too. Let us know.