Ford Focus (64) 1.6 TDCI Intermittent Power Loss

Hi all, been experiencing some random losses of power as I try accellerating.
I haven't been able to take my car on the motorway for over a month since it start happening as it is almost guarenteed to happen when I do 50+mph or go into 6th gear.
I go to put my foot down and notice the car tries to accellerate but its almost as if something is stuck or the power is not being applied, it is still revving but doesn't go anywhere.
I've only recently bought the car and it came with a warranty so I had it looked at under warranty inspection and they found nothing. (Warranty also covers next-to-nothing)
A friend from work also has the same car as me and has had this problem several times, each time being a carbon buildup around the injectors or his EGR valve needing a clean. 
Took it to the garage he had his done at and also had the fuel filter changed to find that whoever fitted the old one had left the old seal on and somehow crammed the new filter on with 2 seals, meaning it wasn't sealed properly at all. Changing this filter did nothing to stop the power losses. He also found nothing wrong with the EGR or injectors.

Today, after avoiding the motorway for over a month, I had it happen again whilst only doing 30mph twice. Almost brought me to a stop before it cleared up on its own again.

Does anyone have any ideas as to what may be causing this random loss of power?
Also I am getting no dash warning lights or error codes from diagnostics.

Thanks.