I'm having trouble with my pressure washer. It's output pressure varies between high and low pressure. I have checked all the possible causes as specified by the owner's manual such as clogged filter, inadequate supply pressure,or pressure control set too low. What is worse, the problem occasionally goes away and everything works fine. But then the problem returns with vengeance. I'd appreciate if any one out there has come across this before. I have never taken a pump apart. I'm happy o have a go, but I don''t want springs and washers popping all over the place by starting in the wrong place and not knowing the pitfalls. With thanks.