Hi. 2001 d4d. Holding a steady throttle, 2000 rpm at 50mph passing through roadworks (every day!) The revs will without warning drop to tickover. They will come back up to 2000 rpm if i put my foot down but lacks power. Normally lasts a few minutes. No fault lights. Changed the SCV's a few weeks ago. Could this be a TPS issue? Getting more frequent.