Mazda 2 diesel engine management light and juddering
 

I have a mazda 2 diesel. When I first bought it, I found it sometimes lost power when driving at 70/80mph and the engine management light would come on. Leading me to pull over and wait a few minutes as it would not allow me to accelerate, in addition to a juddering feeling. Took it to mazda at they could not find an issue.

Recently I found when starting the car in the morning the same problem is occurring when starting to drive.

Does anyone know of this problem, sensor or what to look at as I cannot drive it.