Rich,
They are prone to the terminals becoming loose in the potting of the valve coil, and fatigueing the connection inside which eventually breaks.
With the engine running, very gently "wiggle" the connector on the valve, if it chatters on and off, then this is likely to be the problem.
When they get like this they can be ok one minute then intermittently disconnect, then go back again. You could also check to see if the terminals are at all loose in the valve (pull the connector off and check each terminal with finger pressure - they should be completely solid with no hint of free play.