I agree, there has to be some sort of blockage in the meter movement, this is a VERY common probelm with tachs that were originally designed to last 10 or 15 years MAX!
I think maybe either rust, paint chips or lint/dirt has gotten it there jambing up the pointer's float, or the return spring has broken &/or fallen off. . That said, the modern elctronic conversion kits 9 times out of 10 the old tachs will NOT operate off of the conversion, you have to have the tach totally rebuilt to transistor ignition standards.
Peter Serio
Owner, Precision Pontiac