Question
One of my zones fully comes on, but then cycles on/off about every 10 seconds. All my other 8 zones work fine.  

Answer
Most likely there is a problem inside the valve.  You need to take it apart and inspect the parts for wear and damage, also you should clean out any of the little holes (known as ports) very carefully.  Also remove the solenoid, and clean under it.  You may need to replace some parts, like the diaphragm.