TACH BEHAVIOR!?

["DAVID J. PEAY" <djp4@sbcglobal.net>]

Need some expert advice from you experienced Datsun owners, please.
   
  Intalled new tach cable and fittings last year with restoration project, 
everything has been working fine until last week.....tach needle started 
jumping around and dropped to 0...then back up again....took cable loose at 
distributor and it worked the needle O.K.
  Works fine when starting out and after  several miles or about 30 minutes it 
goes erratic again?
  Any ideas?
  Thanks;
  Dave Peay
  '68, 1600, SPL311..  "Green Hornet"