Hi JMW, I haven't been on the forum for a long-time, but when I read your post, I had to respond.  I had a similar problem with my 82 zx turbo about two years ago.  It was so frustrating, I was ready to sell it for parts (and it is pretty pristine).  A few months after purchasing the car I noticed an intermittent failure to start.  As a result I would bring it to several Z shop in Los Angeles ( no names ), spend a ton of money on it it would work for a few days and the go back to intermittent failure to start.   After going through a similar journey to yours, I made some progress.  I learned that if I pulled the wire and set it next to some metal on the car it would intermittently send a blue spark.  If it sparked, then it started immediately.  No blue spark, no start and no way to determine if it would spark.   Finally I sent it to Mike's Z Shop in Whittier.  He kept if for almost a month.  His first attempt was to rebuild the dizzy.  I disagreed, but wanted to respect his opinion.  He thought the problem was fixed, but wanted to drive her for a few weeks to double check.  The it failed to start for him.  He eventually realized that the ECU had a small hairline crack in it and if the temperature/humidity/whatever was right it would work and then it wouldn't.  He replaced it with a used ECU and for the past two years it has been perfect.  I love the car and it can sit for a couple of months and still starts on the first few cranks.   This may not have anything to do with your cars problems, but I know how frustrating it can be.  Hang in there and good luck.   Larry G