My shooting buddy had a similar problem with a Hawke Vantage - he would rezero it, then shoot fine all day/night. The next time out it would be off left or right again.
It turned out that he would lay the rifle on the bed in his spare room when it wasn't being used and, because something was loose/fecked in the scope, gravity would make the vertical part of the crosshair move.
I found it by zeroing his scope then, holding the rifle tilted, giving it a few good shakes, then shooting at a target again.
There was no reason for it going tits up, it hadn't been abused,