Please help!!!

I have an Audi Q7 3.0 tdi s-line quattro
I up my foot down on the highway. At high speed the glow plug light went on and the car went into limp mode. I stopped and restarted the engine but every time I put my foot down, it did the same thing. Audi specialist garage ran a diagnostic and bought up the following error:
5211 - Fuel rail/system pressure
p0087 00 [096] - too low
Intermittent - not confirmed - tested since memory clear
Freeze frame
Fault status 000000001
Fault priority 2
Fault frequency 1
Mileage 43k

I said I’d take it in a couple of days later to the specialist. Next day the car started to jerk like fuel wasn’t going through. Then it completely cut out and wouldn’t start.

specialist is completely out of ideas. I’d really appreciated some support here please.

its been with the garage now for a month. They’ve changed injectors and tested the following without success:
High pressure pump
Low pressure pump
Injectors
High pressure switch
Low pressure switch
Diesel filter
Diesel temperature sensor
Injector fuel line

They’ve also checked the tank to make sure no pressure is being lost there

I’d really appreciate any help here

Thanks