My 9N starts with good spark and runs good for about 2 minutes and loses spark. It does the same thing consistently every day after sitting overnight. After losing spark checked voltage to coil (6.24 DCV). Replaced the following: Sleeve bearings, points, condenser, condenser insulator, coil, coil wire, rotor, distributor cap, and the resistor. I checked continuity to make sure nothing is grounded. I'm way past plan A. Any suggestions?