Loss of Power :/

[evo]

Guys,

Having some trouble here, and I'm at a loss for what to do...

2008, A8 Sport, 3.0 TDI..., 100k+ miles

Car was driving fine couple days ago, pulled into the drive and switched off.

Went to go out at night and there was just no power. It accelerated and changed gears fine, but just no "oomph". Took it back home and thought I'd let it sit for the night... wasn't in limp mode and went up through the gears to 6th fine and smooth.

Anyway, this morning, wake up to go to the office - and same issue... just no power at all. Super slow to accelerate up to speed, definitely no torque.

Went to the garage, plugged it in, said the turbo is fine, the gearbox is fine and though the engine had some fault codes it was nothing that should give these symptoms. (Codes: 13128, 06256, 12597)

So we considered a blocked DPF... so I bought Forte diesel cleaner, popped that in the tank and went to go and run it on a long run. This is where it gets interesting...

Car gets up to temperature fine, but when on the motorway the temperature gauge comes down from the middle to the first quarter line, and sometimes even below (towards the cold end of the spectrum).

I pull over worried what might cause this, also considering that I was trying to put it into a low gear to keep the revs high and raise the temperature. On local roads, the temperature went back to the middle. But get back on the motorway for the return leg, and the temperature starts to go down again! And on the return leg I didn't even shift down gears incase that was the problem.

What on earth is happening!?

Suffice to say, I still have no power so perhaps the DPF cleaner didn't work, or wasn't really the problem, I have no idea.

Cruise control also won't lock on.

Final note, before all of this ever occurred, as most diesels - under heavy acceleration it used to black smoke a bit. But now, even under heavy acceleration there's no black smoke, not even any white smoke as I expected from the DPF cleaner.

Anyone had this, or any ideas based on what I've described? Your help would be invaluable.

I'm based in Birmingham in case anyone is about wants to try and help assess what's happening here.