John here is the tach in my '70 911T. I think it's an SC tach.
Unfortunately this tach bounces all over the place in normal driving. I've already checked the common problem of having bad voltage spikes but the voltage runs steady. The only thing I can think of is that this tach is not working well with the ballast resistor setup in my car as loren mentioned.
I'm almost embarrassed to post this picture but this was my electrical panel when I first bought the car. It had been sitting in a garage for a few years and obviously was in need of some clean up. I have since cleaned it up a lot but it's not quite up to the standards of Shaun 84 Targa's restoration.
I also have a '73 tach that I bought but it apparently reads low. Based on what you said in your first post it won't work in my '70 so I need to keep my eyes out for a good '70 T tach.
Oh and here's an in progress shot of the engine room...
