Looks like the bank of solenoids isn't getting enough voltage.  Probably either a bad ground or corroded/pinched positive.  Seems to me that they get unreliable when the voltage at the valve terminals under load is below 12.5v.  I doubt all three solenoids would be going bad at the same time, unless you just started spraying something really thick and goopy that settled overnight and is gumming up the valves.  98% of my solenoid problems involve bad power, although I've had a few of the poly body 144s die on me.  They would only partially open for a while and run really hot before not working at all.  It was really easy to test them though- just run an ohm meter across the two connectors on the valve after pulling the wires and see if they're all the same.  If memory serves the "good" reading is around 6.5ohms.  It might be hard to justify popping $700 or so for three new ball valves if fixing your solenoids is cheap and easy.