First off, I'll link a video of the issue and tell me what you think it is. This or next weekend, I plan on changing the Spark Plugs, and doing a Coolant Flush and maybe a Boost Leak test.
It's done 258,000KM so I plan on doing a full service and giving her plenty of love in the hopes of solving this issue.
https://youtu.be/JnAVwieuGtk
So that cutting under boost is the issue, only happens above 3,500 RPM. If I don't put my foot down at all then it doesn't have a problem at all and drives fine. But once I put my foot down, give her the beans, it sputters, cuts for a second, accelerates slowly through the gears up to speed hiccupping as it goes. When it hiccups, it'll blow a bunch of black smoke out the back.
It's been to two different mechanics and they both couldn't figure it out, so I figured get it WOFed and book it into the place where the transmission had been replaced (done at 245,000KM).
Recently had a new fuel pump installed because the mechanic wasn't getting anything from the return fuel line. Called him yesterday, and he said once he'd installed the new fuel pump it was getting fuel from the return line.
I have EvoScan so I can upload my EvoScan logs which sensors should I log as I do my runs?
What should I be checking in the engine bay? The mechanic checked all the turbo piping and said that was fine.
Any Mitsi/Turbo specialists down Dunedin sort of area?

I'm a new member of ClubVR4 and I found my chassis code in the forum post that mentions Super VR4s, which I am delighted about because I had my suspicions she was special.
Any help would be appreciated