Why is my Volvo V50 losing power?

My Volvo V50 (2006) keeps slowing down on long stretches of the motorway, after an hour or so, and displays a sign saying "Engine System Service Required" on the dashboard. No garage, mechanic or anyone seems able to help.

It only happens sporadically on long journeys - one time out of three - but it slows down to such a point that I need to stop, wait for half an hour or so, and then usually it rectifies itself and we can continue on our journey. Any ideas on what this fault could be?

Asked on 21 April 2015 by Henry

Answered by Honest John
Presumably this is a diesel and the diesel particulate filter is clogging up with ash.
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