fender fm212r guit amp volumn jumps on its own

SInce its a solid state amp there are most likely a few issues you maybe facing. First I'd check for loose solder connections. Its very common for the from panel controls to come loose after years of use, but its also very common for the interconnects from board to board to have cold solder joints too. Occasionally too the headphone jack or the effect send is tied in series with the signal and either that jack is dirty or loose. The only way I've found to isolate a problem like this is to open it up and tap around the board gently to moderately with the plastic back end of a screw driver when the unit is on(BE CAREFUL not touch mains voltage). If the unit exhibits the problem then. Its definitely a loose connection or similar somewhere in the unit. If not, then you maybe facing a transistor problem and those issues are best fought with a scope, multimetter, and patience. Let me know if this helps. Thanks