Nothing else is on the circuit with the sprinkler.
I have never looked at the controller while it was doing this sprinkling! But I will look. We have been having serious rains this week, so have not gone outside to dismantle the system yet.
Using the "grasping at straws" method:
is there anything close to this can could interfere? High speed motor? Pump relay? Welding? (really stretching on that one).
Something that might come on and cause interference even if not on the same circuit?