I used the diagram and part parameters you listed in your post tonight, and my negative spike problem seems to have gone away. I can repeatedly turn on and off the car, and the empeg behaves.

The only remaining oddity, is that after turning off the car, and pulling the key from the ignition, I can still turn on the empeg, which is unexpected. It's in standby mode like it should be (shutdown timeout is set to 300 seconds), but I guess I was surprised that I could turn it on during that period. I assume this is normal?

rjf&