This is the 3rd time I am typing this - my computer keeps deleting it before it posts...
Anyways, all of my problems with Di2 have had to do with the multiple modifications to the wiring harnesses - specifically poor soldering and shrink wrapping. I would go to a shop that has a new main wire harness and front wiring harness and plug each into your system individually to isolate with harness has the problem. Then either swap the harness out or re-do all of the connections in the bad harness.
I have had to do this a couple of times where a crappy solder connection came apart. Annoying, but now I am a master of soldering. OK, not a master, but the connections are now great. The key is using flux to prep each wire before soldering. Wicked strong connections that way.
Have you ever wondered if there was more to life, other than being really, really, ridiculously good looking?