Installed new coil, now the tach acts funny

^^NO!!^^ That I doubt very much will work. One of the wires in the dist. harness goes to the "system ground" in the module, and the other is the "hot" or signal line for the pulse. The way to figure that is to disconnect the connector and take up the bay harness end, not the dist. end. The terminal that is the signal is the "bare" terminal. You could try it, I guess....."Rig" wrap a small gauge wire around the bare terminal and temorarily re-connect it, hook that to the tach

"It might be" you are suffering EMI/ RFI---same as magnetic or radio interference--that is falsely triggering the tach

MAKE CERTAIN you have the correct wires from the igntion connected to the coil. Make CERTAIN that if the coil has a radio supression cap, that it is connected properly to the coil+

What color are the two coil wires? If you are unsure, DISconnect the coil negative. Hook a voltmeter to coil + With the key in "run" the coil + should show 12V if correct.