There is definitely a bad connection somewhere. Whether it be the wiper of the knob potentiometer, or the input jacks, or the cables, something is not making proper contact. What were you wiggling near the end of the video? That seemed to have a major impact on the sound. My personal guess is that you've got some cold cracked solder joints on the input jacks. Best way to fix it is open the thing up and re-flow the joints with a soldering iron. I would need to see photos of the inside though, where the jacks are soldered to the circuit board.