Hello Everyone, after some help trying to figure out the issue with my Mondeo. It is a 1.8 TDCi 125BHP 2008 estate manual 6 speed.

 When travelling at 65/70 mph on smooth level roads either in cruise or on in "manual mode" where there is very light throttle the car will judder sharply (actually feels like a bad imbalance in the wheels but these have been checked), although the speed remains constant the RPM will bounce up and down 100rpm at a frequency of about 2 seconds, soon as you increase or let off the throttle it is back to perfect smooth running. At low speeds and accelleration it is perfect and doesn't smoke while running or misfiring as I can tell.

 So far checked for;

• air leaks, found hose after MAF wasn't on perfect, refitted - no change 
• Blanked EGR - no change, returned to normal
• Changed glow plugs - not related really but all was failed 
• Using ForScan perform PCM reset - no change
• Using Forscan performed Knock sensor learned values reset - but more knocky to start with but settled down - no change
• Trended it while fault preset but not seeing anything obvious other than when it is happening Barometic air pressure is bouncing around 0.5KPA
• No pending or present DTC are shown of ForScan and no glow plug lights or warning lights on Dash

 As this is bugging me a little now and before start throwing money at things like injectors, MAF, MAP when no obvious issues seen yet got any ideas what to look for next?

 I'll try and send a copy of DTC's and trends soon.

Additional information I have also replaced fuel filter, air filter and both timng belts and checked the timing. Thank you :)

Sounds like the classic case of a boost pipe having a split in it..The rubber pipes have a mesh lining that holds the split together until a certain boost pressure forces it open, resulting in loss of boost, hence the juddering. Splits can be quite elusive and sometimes its easier to remove the  pipes completely to check them, but any oil on the surface is a tell-tale sign of a split. 

Morning, Thank you, when I get a spare mornnig I think i will take all hoses off and inspect, had this before on another car with a pin hole in boost pipe but this would go into limp mode when going up big hills.

 Trended it on way into work today as it does it for last 2 weeks at same area, but today it was fine! attached is screen shots, firstone is today while it was working fine and other is from a few days ago when it did it but only minor compared to other times. It was cooler this morning, so could be why?

Yesterday I disconnected the MAF and drove it, no issues with the vibration either but was quite down on power as expected, will try again on way home tonight.

Apologies on the trends, not very good at seting up the formatting on Forscan yet 

Hi, the colder morning start makes sense as the rubber pipes will be stiffer and need more boost pressure to. open up a split until thoroughly warmed up again... and they do get hot.