Thanks for the replies guys. I’m using Richard Zimmerman’s program and it seems to be doing a fine job of avoiding multiple simultaneous programs.
I did check all solenoids and all were 25-27 Ohms or higher, so was still scratching my head. Then yesterday morning it was unresponsive again so I figured the P/S died again. However, it was just seemingly locked up – power cycle and it came back to life. Then it hit me that Wednesday is my watering day (were in a pretty severe drought here in North Texas – can only water 1 day ever other week), so I got to poking through the logs and I see that starting zone 3 is the last entry, even though there are many zones after that in the program. So I re-measured the solenoids last night and now zone 4 is showing 13 Ohms. By my math, that means its drawing somewhere in the neighborhood of 2A, which I’m sure its not happy with. It must have ended zone 3 and started zone 4, and the power drain was enough to lock up the Pi before it could update its log file.
Now I just need to get to the bottom of the low resistance. Hopefully its just a bad solenoid like Ray suggested and not a wiring short that only shows up when the soil gets wet. Still scratching my head on why the resistance varied so much from one week to the next.