did you respace the frame (like a 130 spaced road frame to 120 track width)

if you did a drastic resizing, it could be possible that your dropouts aren't parallel (so the faces where the nuts tighten are not parallel between left and right). an un-parallel dropout could be the result of some sort of crash too.

if they aren't parallel then the slight change in distance between the dropouts over the length of the dropout could make proper tightening of the nuts impossible and can cause the creeping you see.

if this doesn't make any sense, it's late and i'm tired.