Horizontal Position Unstable

Question

Running monitor at native resolution, and sometimes it would switch on shifted to left, and other times shifted to right 10 pixels or so.I pressed "auto" to centre it each time it was out.Came back from holidays and now it is left 20 pixels and will not "auto" correct any more. I can manually adjust it back.What is the cause of the "amnesia" regarding my settings, and why won't the auto correct work anymore?