I have the latest P-Mag update, and just sent an e-mail to Emagair yesterday about one of the PMags not being "on" after startup. It happened to me yesterday at run-up prior to flight. I was doing my mag check, and right when I killed the Right Mag, the engine started to die. I re-engaged the left mag to keep the engine running, then I removed power from the left Mag. The engine continued to run smoothly.
So, I shut down the engine and removed power. Then restarted using normal procedures. Both P-mags were operating normally this time. I have my P-mags wired the same as Bill Repucci, but with a switch instead of the stereo plug.
I haven't heard back from Emagair yet. Curious about this one. I will admit that my grounds are run to the ground buss, but I do not believe this is an intermittent ground problem.