Solution to unresponsive volume buttons?

ZTE ZMax Z970. The volume buttons are randomly responsive, will work for an hour or two before I have to go back to adjusting my volume through settings. I'm not sure if this is a hardware issue or software. Possibly related: adjusting the headphones jack when something is plugged into the phone causes the Google app to open up and listen for commands; sometimes, it will suddenly increase the volume in increments and I have to manually change it back. Where would I take it to be repaired?