I think I remember having a jumpy tach and speedo (just jumps a little off the peg, in time- jump, jump, jump, etc...) once upon a time in an SC. The '81 I am going to pick up next week has the same issue. I think I remember it being a vaccum leak or maybe something more costly like an alternator. Does this sound like anything you guys have experienced? Thanks in advance!!!

 

IME, the only time the tach and speedo get jumpy is whent the voltage is very low. I suggest you check the voltage in the system with engine off and engine running to check the alternator/battery function.

 

I've heard there is a damper circuit in the tach that when it fails can cause a jumpy needle, but htis wouldn't have anything to do with the speedo.
Check for good ground connections as these cars use a series connection (my 84 does anyway) to provide ground from one gauge to the next.
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Check your voltage regulator

 

Could be a number of things, including those mentioned. Had this same problem once... went away after removing, cleaning, and establishing a good ground.