Sorry @scifi, I guess I deleted my last post while you were replying to it. I called the manufacturer and they knew the cause of the issue and gave me the solution, so I deleted the post thinking no one had seen it yet...
For those that didn't see the post, I replaced the points with a Pertronix Ignitor 3 unit... Runs fine, but the tach was reading about half of what it should be. I checked their website and they said *if* your tach reads too high to add a 10k 1/2w resistor in the signal wire between the coil and tach. But nothing about reading too low... Turns out the resistor is effectively a signal stabilizer and should fix my issue as well... I should have verification over the weekend...
@scifi do you have your old veglia tach? The fiat 124 spider tach is a veglia and the same internals as the Mangusta tach... Have you ever been into the Veglia's? Pretty simple stuff... 5 of our gauges are pretty much all the same, just a different face plate...
MH