I have a Hitachi ex 220 and it was always a strong machine with plenty of power then all of a sudden the pump kicks off whenever you move it. Does so in all directions and with tracks whenever you do anything. Wait for a couple seconds and it re-engages. Is there some sort of high pressure regulator sort of something that might be acting up and cutting it off whenever it thinks it's working too hard? When it moves it has plenty of power.
What is the Dash series of Hitachi? The problem you are stating sounds like the valve plates have gotten scored and are being blown off the piston and barrel assembly when ever pressure comes up and the oil is forced into the damaged areas.