I've had her a loooong time. I turn it on, all is well. Within 5-10 minutes, the sound gets darker, and the energy of the amp (the drive, the heat, all the stuff that makes it sound real) fade away. It's kinda like tubes going bad, but well... If I turn it off for half an hour, it'll revive, albeit briefly as before. Has anyone experienced this? If so, what was done to correct it?