Why does my Renault Espace Race lose power?

Intermittent problem over years, often loss of power on a long journey, especially when having been running well then had to slow down. Sometimes revving when stationary has cleared a load of black stuff, but today smoke has been coming out and black stuff. Garage has changed exhaust recently, don't think dirt being sucked in. Year 2001, it's diesel

Al
July 2011