If the chuck is flat, and is perpendicular to the column, can you swing the spindle across the chuck while watching an indicator at the chuck surface ?
Might see a deviation during / after the "step down" through the course of the swing.
May be some wear in those mating surfaces ?
Eliminate the chuck and test grind a block clamped directly to the base.
I like that style of grinder, wish I had one.
See ya,
good luck,
Russ