Shimano Dura-Ace 1st Generation Brake adjustment
Your spring is not right - its one or more of


bent unevenly
lost springiness with age
pinched 
mounted not square in the retainer.


I've tried to show the difference I see here:



To be honest unless originality is a big thing, consider moving to a dual pivot brake caliper.  The old single pivot side pull caliper was always sub par.