I have a SunPro tach in my daughter's '69. It works great with the Pertronix Ignitor in a stock distributor with an Accel coil.
I have a SunPro tach in my '66 and it works great with Pertronix Ignitor, Pertronix Flame Thrower coil, and MSD-6AL. The tach, however, MUST plug in the MSD-6AL if so equipped, or it will bounce all over because of the multiple sparks.
I'm running the sun tach 2 with the petronix 2 and a flamethrower coil in my 66/289 with no problemo. Maybe the resistor wire is causing a problem of some sort. I removed mine for the flamethrower coil... good luck.