man, valve stuff is a real problem. tube fatigues, blownups, u should replace them regularly... in this case, its sounds like voltage anomaly. in the other worlds, anormal voltage instability. u know, when technicians checking, everything was cool, but another day, ZAP! maybe there's weak contacts or shortcuts exist...

i think u shouldn't use it anymore. valve amps are hi-voltage devices, and they can ZAP your axe's ground... that means u'll get ZAP when touching strings...