triton opal shower?

My triton opal shower has started going hot and cold. The water pressure light stays out with only the heat light on as it should be. When the water changes temp the pressure stays the same from the head. I have checked the pcb and resoldered a dry joint at one of the relays (thought i had found the problem). I also cleaned the contacts on the relays as they looked quite black. The pcb looks ok with no water damage or any obvious damage. The shower still does the same and starts off warm then after about 30 seconds the water starts going cold then warm again.

Any suggestions