Heavy sputtering around 5k rpm when accelerating hard

[Reeceyy]

I did make another thread earlier this month but the problem persists.

When giving it full throttle, the car sputters around 5000 rpm as if it were bouncing on the rev limiter. When giving it gradual throttle, it doesn't happen. Only when accelerating hard. It's very frustrating this. 
I've changed the following. Bank1/sensor1 oxygen sensor. Checked camlobe and cam follower for wear. No wear and cam follower was replaced. Changed the piston in the HPFP to a vis one. The car was fine when all that was changed. I believe the oxygen sensor made the difference. But the problem is back! And this time it seems worse than ever. A mechanic said he graphed the fuel pumps and turbo and they are all running fine. I'm lost for what this could be. He said that he reckons it could be a stretched cam chain but I'm not too sure. It's a lot worse when the engine is hot!!

The car is 290bhp stage 2 with S3 intercooler, downpipe, Vis piston and ramair induction.

Please, if anyone has any idea of what this could be, I would appreciate it greatly if you could let me know.

Many thanks.

[mjmallia]

Have you carbon cleaned the intake valves?

Spark plugs?

Fuel filter?

[ZoliWorks]

Log lambda control with VCDS. I've got the same issues past 5k rpm because I'm running K04 boost on k03 injectors and the ECU is trying to prevent engine damage due to running too lean. My lambda is maxed out at 25% so I've got constant loss of power as the ECU is trying to figure out what to do.

[Reeceyy]

Have you carbon cleaned the intake valves?

Spark plugs?

Fuel filter?

Fuel filter was changed last month.
Spark plugs changed to NGKs last month
Coilpacks changed too.
I've also changed the low pressure fuel sensor the lower one on the HPFP which is on the fuel rail. None of this made a difference.

I haven't carbon cleaned the valves yet though but I've been told that carbon build up shouldnt be that bad because it's petrol and not diesel.

[Reeceyy]

Log lambda control with VCDS. I've got the same issues past 5k rpm because I'm running K04 boost on k03 injectors and the ECU is trying to prevent engine damage due to running too lean. My lambda is maxed out at 25% so I've got constant loss of power as the ECU is trying to figure out what to do.

I suppose it's more or less the same problem as what I'm going through then. The only difference is that I have k03. The map is maxed out though. What's the cure for this then? Have you sorted the problem out?