First, thanks for your quick reply, Johannes.
Second: I think it is my problem.
The problem seems to be the plastic screw-on shield on the cable connectors going to the solenoids. As Johannes noted, everything in my setup besides the automat is DIY. I used 20 gage solid core copper wire, which is stiff, it does not drape easily. These connectors are soldered to the copper wire and use a thread on plastic insulating cover. (Picture attached)
When the plastic sleeve is screwed on tight, it apparently keeps the connector from seating properly in the automat, and the false triggers happen. If I loosen the sleeve, which lets the connector sit deeper in the automat, I don’t see the triggering problem.
I will try reshaping the plastic to see if that works. Eliminating them entirely is a bad idea, but there may be some other option.
There are more expensive, and bulkier, solder-less screw type connectors that have a longer metal barrel and do not seem to have the same problem.
For the record: I am using a 9v 1a power supply. I was using 5v solenoids, because that is what I had on hand. The recent attempt was with 12v solenoids, but still using the 9v 1a power supply.
Thanks,
Vincent