Question [Q] Volume buttons trouble

Today I got really weird problem. My volume buttons worked good, but now, the up button when I'm holding it, mutes the phone. Single click sometimes makes louder, sometimes it acts like down button, and sometimes does nothing at all. The lower button acts similar. The camera and power buttons are working like new. It happens on 4.1 and 4.3, so I think it's hardware problem. But what caused this and how to fix it? Please, I really don't want to give this phone to service!

Timeline of all my phones click here