Bongo Fury

My Diesel Auto Bongo has an idling problem. The revs settle on tickover but often rise to 1800 then suddenly come back down to 800. Have renewed the Throttle Position Sensor but problem still exists. Intermittent but worrying as it makes gear shifts jumpy and plays havoc with mpg. Can anyone help please?

dont know but the banjo filter could be a possible ( based on other posts ive read in the past )