1990 Mustang GT, erratic tach, stall

Hi, I have a 1990 Mustang GT with over 220,000 original miles. The past 7 or days, I've been driving along, and all of a sudden the tach needle will drop suddenly and with in a split second jump back up to where it was before. If I have my foot on the gas, the car will jerk or buck for that split second until the tach needle jumps back up. If my foot is not on the gas, the tach needle will still drop out (down) and come back up but the car won't buck or jerk. This has not been happening all the time, just once in a while, but got worse Saturday and Sunday. Also, while stopped at idle, the idle would once in a while fluctuate (like the idle air bypass valve was going bad).

Because it was old, Sunday I replaced the ignition module. Monday the car drove overall ok, but "hiccuped" a couple of times.

Monday driving home on the highway, the tach needle started the "hiccup" again when it would drop suddenly then come right back up. Then it started doing it bad, then died while driving. I pulled over, and tried to start back up, but it just turned over without starting. After about 20 seconds of trying, the entire electrical system was dead. No headlights, no instrumental lights, no dome light, nothing.

Something that was strange was that the car did start turning over without the key in ignition and me turning the key.

The AAA tow truck towed it home, and when we got there the driver attempted to jump it. When the jumper cables were attached to the Mustang battery, it started turning over without the key in the ignition and me turning it. After a few seconds, the electrical system died out again. I removed the negative battery cable and measured the battery voltage at 4.6 volts.

I did have the alternator tested today, and it was fine. For what it's worth, the ignition coil and the starter relay (by the ignition coil) are both original equipment. Also, I did remove the ignition coil, and measured the primary resistance. It's measuring about .6 ohms.

An ideas?