I only have about 12 foot from the controller to the solenoids on 16awg multiconductor.
It seems they work fine until it gets hot and super dry then they start dropping like flies.
I have long run times on the zones and hour 20 mins, and the box is full sun and located in a dry non-irrigated area next to a parking lot. It really wouldnt surprise me if the box was 160 degrees during a run cycle.
Thats why I am assuming heat.