Weird Issue, Not Sure If Misfire R32 Gtr

[SiR_RB]

Now my car doesn't misfire giving it stick in 1st, 2nd, 3rd gear all the way to redline, but when I'm cruising in 4th gear and lay into it, soon as the gate cracks open I get this unusual jerking/misfire/popping action

Not sure why it doesn't do it in other gears but only in 4th when ramping it up to max boost?

Thought my old splitfires were tired so I put brand new ones in, and fresh plugs at .8mm,

Issue still there

Checked and cleaned all the earths too

New alternator

Last time i gave it a hit in 3rd and into 4th at high revs it didn't do it, but I wanna test that again and see

I'm stumped, any ideas ?

[SiR_RB]

Double post