got to be a timer problem of some kind
cant be pressure sw as this would not operate until unit full
its a 2008 machine
its there anything else running while the valve is operating
my only thought was board power supply. at this age they probably used a capacitor to provide power to the board and these fail over time till there is not enough power to do some jobs. since they are cheap crack the timer case and see if it has one and swop it
Dave