Intermittent loss of power

I've been having a problem with an intermittent loss of power which always occurs when accelerating up a hill...Had air mass meter replaced about 3 months ago which seemed to do the trick but has now come back again. Had diagnostic check at main dealer which eventually found a fault code 5051 "Intermittent fault on turbo" - they have recomended new turbo at £1100 ! Turbo hoses all checked ok. Am considering again replacing air mass meter to see what happens?
Any ideas anyone?

What sort of auto is this?

Its a Seat Alhambra MPV with the VW 1.9 Tdi (115bhp) - not sure you have this model in the states - its the same as the VW Sharan if you have that one?

Is this a PD engine or older? How old?

I'm trying to understand the layout of the engine. Wastegate or VNTturbo?

In all of them, with proper age, you should look at, possibly replace, the small "vaccum line" that goes to the turbo from the N75 valve, as well as the other two lines from the N75. They may be blue, red and black. Often they crack and cause a deviation in the reading of the boost level, which confuses the ecu so it puts you into limp mode.