I also tried with the diesel tank filler cap loose should there be a vacuum relief valve issue but no change.

As my car is rapidly becoming worse in symptoms similar to yours, I have yet to check for air in the diesel entering the tandem pump, it occurred to me that with temperatures near freezing perhaps it's a temperature sensor gone bad as it runs fine when warmed up.

 

How did you get on with your mechanic?

I fixed my issue: