I don't know the ins & outs of this particular engine, but most turbo diesels use a variable vane turbo. What usually happens is that the variable vane mechanism occasionally sticks due to a buildup of carbon from the exhaust. They nearly always stick in the full boost position leading to the overboost condition. The ECU then reverts to reduced power to protect the engine. Switching off/on the ignition thus rresetting the ECU is usually a short term fix, but the turbo will need to be taken apart & the variable vane mech cleaned thoroughly.

Edited by dieselnut on 07/06/2013 at 11:46

Hi, I've just had a similar problem in my 2006 2.2 CDTi whilst towing at 55-60mph on the M5. Not long had vehicle and have towed twice before. Was using cruise control and changing from 6th gear to 5th to see difference in fuel consumption readings when lost power and engine management light came on. Was recovered to services by AA, fault was turbo boost pressure too high. Fault was cleared and could not be reproduced by AA engineer. All settings were well within perameters. Towed rest of journey and return journey with no problems, avoiding using cruise control and avoiding 6th gear and only using 5th over 55mph. No repaeat so far.

Have you had any further problems with yours whilst towing UncleRob?