I’ve had intermittent problems on my controllers. One, I discovered that I was using a new Raspberry PI 2 with a new WiFi dongle (with antenna) that was using more power than the tiny Edimax USB adapters I normally use. I switched to a bigger power supply unit, and the problem went away.
The other is an intermittent problem I have with a garden controller that just goes off line periodically. I just come back later, and things are working again.
Since I am looking to expand my WiFi area coverage and traffic, I’m thinking about replacing my WiFi network with a Open Mesh professional grade network. More expensive than home routers, but if I get good, reliable coverage, it will be worth it. http://www.open-mesh.com/