When I turn on A/C,  gimble goes through self test and is OK,  tilt OK and camera up down works OK.  While flying after a few minutes, camera begins to oscillate in tilt mode but up/down still works OK.  I have recalibrated the gimble,  never crashed it and it has previously always worked OK.  It has been idle for a couple months but no other changes.  Could this be a software problem or a mechanical problem?

May I know which drone you are referring to? Please check if the anti-shock balls are damaged, if yes, you need to change it. Or please provide me a video of this case for a better confirmation.