Factory Tachometer

[biscaynesix]

Hi all, my 1976 C/20 454 3-speed manual + granny gear has full factory gauges and the tach. The tach indicates properly about 30% of the time. When it does not indicate properly, the needle runs counter-clockwise. No rhyme or reason WHEN or WHY, BUT, if you are running on the left side fuel tank and QUICKLY bump the switch to the RIGHT tank, the tach will momentarily read properly and then go back to the incorrect reading. Any ideas? I have checked the wiring to the tach from the dist lead and 'looked' at the tack when i had the gauges out to clean the lenses and nothing looks amiss......