Installed new coil, now the tach acts funny

I cannot stress enough to check for possibility of some sort of electrical interference, EMI/RFI. The destructions were talking about triggering it off the pickup which is a VERY low voltage, low power source. This means the tach is quite SENSITIVE. A long meandering pickup wire to the tach may be too close to some other wiring which is inducing interference, including secondary (spark plug/ coil high tension) wiring.
Check the thing is properly grounded, and that it is not grounding through the lighting circuit.

Try to "dress" the pickup wire close to ground, along the column, near the firewall, etc, which is as clos as you will get to a "ground plane" and will help reduce noise.