Help with a miss and voltage drop

Put an analog voltmeter on the + feed to the ignition from your relay and make sure the voltage is not dropping out there. If it is, then look to the ignition switch and the feed to the relay, the relay etc. I'll say it again.... your initial symptom description jives with the key switch dropping out the power to the ignition and to the charging system. (You could also use your dash meter temporarily wired to various spots to monitor this.)

Also borrow an accurate meter and check the voltage to the battery when at fast idle; your dash voltmeter is only so accurate. A good way to check a meter for accuracy for automotive uses is to measure the voltage on a fresh, unused 9V battery; they are consistently right at 9.6-9.7 volts if new and fresh.