Tachometer problems

My tach. was not working, so I installed a new one from WCCC. It soon became erratic, swinging erratically. WCCC kindly supplied a second one, which did the same thing. I took a video of it but can’t download it. The wiring is correctly connected - I have double checked. Could it be RF interference? Any suggestions would be appreciated. Thanks.

Is this one of my (RCCI) converted tachometers?

Are you using some sort of multi-spark distributor system, such as MSD? Unless you use a converter module, most are incompatible with the Ford stock tach.