expressions 500s model 53-673001. starts /stops when switched on?

I have a redring expressions 500s model 53-673001
when switched on it starts/stops continuously. I have checked solenoid with meter( 3.9k olm) and checked inlet filter which is clear. water pressure is ok. shower looks in good condition with no signs of wear / water ingress although ther was some signs of water on on / off switch when i removed shower cover. any ideas?