Chances are you input jack is breaking free from the solder holding it to the board. Unplug Amp and then speakers, Use philips head to remove amp head. Also unscrew the knobs and nuts securing Potentiometers and 1/4” Jacks. Once you have the amp head separated safely then analyze input jack. Suction and or whick the old solder off. Use soldering iron and solder of your choice to resecure the input jack to board. (You can even skip the desolder part and just hit the joints with a fresh bit of solder). While the amp is open go ahead and resolder any other solders, but more importantly you can also get some contact spray and spray all the potentiometers while turning the knob to clean out any residue. If your volume or gain pot (or others) are dirty it can be responsible for connectivity issues.
after that verify that the speaker cable going to the amp board look good. Then reassemble, be sure contact spray has dried, and then plug in and play (hopefully it sounds new again).