A few times recently my focus TITANIUM TDCI 1560cc DIESEL at around 70-80 mph after travelling for about 2 hrs suddenly decides to do what I can only explain as lurching.

Firsr time it happened was about 2 weeks ago on the motorway and there are no lights that come on, the rev counter stays steady and I experience a very weak lurch and accelerating kind of motion, no real engine sounds or knocking, pinking etc, infact only smooth deceleration and speed restoring, no un controlled excessive engine speed either. This happened about 8 times in succession and when I dropped to 60 it then ran as normal. After a few minutes at this speed I was able to bring the speed back up and continue my journey. My local very good and friendly garage put a diagnostic test device on it and it seemed to show the DP filter had tried to 'recharge/reset' itself. I can't remember the exact words the mechanic used.

It has a brand new (12 months or so) DPF and I run it regualrly about once a month in 4th gear to burn of the carbon, because most of its werkly 2xdaily journeys are about 7 miles each. I know, I know. LOL (Wifes work car). Should have got a Petrol one etc for short journeys.

However, around town speeds I have none of these lurching symptoms.

Same happened a few ago on motorway again, even in cruise control mode or if I am using the accelerator foor pedal.

Today after about 2.5 hours same speed on cruise control it happened again. So I turned the radio off and as it decelerated I could hear a kind of slipping sound best described as a slipping fan belt but no squeak more like a rubber belt faintly whooshing on a plastic pully is what I had pictured in my mid. The faint noise matched times the car decelerated and eased off when the speed smoothly came back. When this event begins it usually lasts about a minute or so and I did notice today that whilst the event was happening and I purposely pressed the accelerator it exaherated the lurching feeling. But this time as I eased off the speed the noise went away and the lurching stopped.

So, is there some sort of fuel pump that may be slipped when its belt gets warm and is losing a bit of friction on which makes the engine lose power and then pick back up again. Its a bit like its retarding and advancing but with only a faint belty plasticy very faint rubbing whooshing sound which wasn;t even loud enough to capture on my phone over the road noise.

Update_ 26 July 15 - Stumbling actually very low pitched fan bel type creech and lsight burning smell, opened bonnet with engine off and sooty deposit in turbo area and pipe front facing and seen clearly from the front. Turned engine on and under idle no smoke or hissing screecing noises, no coloured smoke from exhaust either anytime!!!

Any help would be really appreciated.