tach resistor would be a long shot
imho.
I went through what sounds like the exact diagnostic with a Mallory Unilite a couple months ago. I had a combination of a bunch of things wrong. I'd correct one and another would pop up. First, I was trying to fire the car without the tach. I put in a jumper, first problem solved. Next, the car didn't have a neutral safety switch. Easy fix. Next, the Mallory was wired wrong. Another easy fix. Next, the rotor and the ignition module in the distributor were bad (don't jump start your car or charge battery with the Mallory distributor connected to the coil). Next the ballast resistor (you do have one, right?) had a hairline crack and would intermittently work. That was the tough one.
The Century Performance link I sent you has a diagnostic for the distributor and components and a wiring diagram. That helped me find the toughest problems.
Let me know if I can help...