my mesa single rectifier solo head has been fluctuating in volume....mostly getting softer in the middle of playing, a slight tap on the housing usually corrects this.

but i doubt in the long run hitting the amplifier is good practice lol.

what could be causing this? how do I correct it?
Could be a "cold" solder joint somewhere. These are often intermittent and corrected for a time by jarring/smacking the unit. Eventually though it could fracture all the way.
Moving on.....