Rich, if you need two more cents this is what I think and in this order.
Losing all power indicates a faulty battery lead connection. Clean and reinstall all battery lead connections and ground straps.
Being able to jump the car and drive indicates that the battery is bad and is running on the alternator. Take battery to Sears or other for load test which takes approx 45 minutes. Long term storage, like thru winter, or a parasitic draw on the battery will ruin a battery as it leaks down the voltage and causes some kind of chemical reaction (so I have been told). Usually I start my car every month and have never had a bad battery. However this last winter I did not do that and I had to replace the battery because it would not hold a charge.
Battery charged to 18 volts indicates bad regulator. If your alternator is single wire the regulator is built into the alternator itself. Your battery should never be charged over 14.7 volts. A volt meter will tell you what is going on there.
FYI Your MSD box came with a fused wire so it depends whether you used it or not. It is not internal. MSD tech has told me that an MSD box is either working or not. There is no intermittent operation possible. Ignition switches can cause all the above with the exception of the over charging. Found that out with a 69 Cuda that was previously owned by a maintenance guy that had a heavy set of keys. Had a car that would often just click the voltage regulator or even turn the motor over for just a moment. Often times rocking the car in 2nd gear would allow a good start and thought the starter had bad brushes. As it turned out the rocking of the car would reground a bad negative cable that had shorted in the cable somewhere.
The important thing about diagnosing electrical is to be methodical, having somewhat of a plan and ruling out the easy stuff first. Good Luck
