hp indigo 5000 vacuum adjusted to high but it kkep going from 0.75 to 0 every 5 second

I have problem with my hp indigo 5000 vacuum; even changing to high value, the value keeps bouncing from 0.75 to 0 and from 0 to o.75 after the machine stops. I checked the pipe from the vacuum to the perfector -- looks fine the gauge in the vacuum reading is 0.82
i switch the perfector with old one in good working condition but i still have same issue . Any idea?
Thanks