hi there,
I am having the exact same issue as described, in general the car (x-type 2.0d 53plate) drives lovely upto around 35mph then as soon as it comes to accelerating past this getting upto 50-60mph the glow plug light flashes sometimes accompanied by the engine management light and a low pitched rumbling sound, as soon as i stop and start the car again the glowplug light is gone and usually engine management light is gone by the next day. did you manage to rectify the problem