voltage drop under load
The annoing thing with troubleshooting these kind of problems is that when the systems is "idle" (no power to soilenoid) it likely gets the normal battery voltage.
Only when the solenoid starts to work and draws current the voltage drops.
If you are "lucky" it fully resets the computer (the controller is a simple computer running a single program) causing it to restart every time.
When the voltage drops below what the computer needs to work correctly but remains above the voltage of a full reset then you could end up with random behavior like described here.