If it only happens after slowing down after a long period at steady (Motorway) speed, it would seem to point to fuelling. With conventional mechanically controlled diesel engines, the mechanical governor in the fuel pump would cut the fuel off completely "on the over run" when the engine was slowing down, reinstating it when the engine speed dropped below a predetermined value, generally about 850 rpm or idle speed. Modern electronically controlled engines, both diesel and petrol, do the same and, with my 2.0 TDCi the fuel "cut in" point when the car is moving is about 1000 rpm and is quite noticable as the car will quite happily trundle along with no throttle input after that, in the lower gears at least, given the right conditions. It would appear that your engine is having its fuelling cut, but not reinstated when it's slowed down. It could be an ECU fault or, more likely, a problem with the engine speed sensing element which may be giving false readings after having been at a high temperature for a prolonged period. If the engine is OK after it has cooled down, and in normal day to day use, this may well be the case. It's not unheard of for electronic components to throw a hissy fit due to prolonged heat exposure and then be perfectly normal again when they've cooled down. This may be why no fault is showing up when the system is interrogated by the diagnostic machine. If the speed sensor is telling the ecu that the engine is going faster than it is, it will continue to cut the fuel, as that's what it's programmed to do. This is a bit of a stab in the dark but that's how it seems to me.
2008 2.0 TDCi Zetec