Trying to help diagnose a part throttle boost issue on a MKIV 1.8t.  This is probably a shot in the dark but maybe someone can help.

It's not an issue with the tune as the issue persists at WOT on a stock file.

It seems as though any boost above wastegate cracking pressure is uncontrollable.  The car is also using a MBC to control boost spikes and then oscillations that would ensue at WOT.  Any attempt to boost above 5psi, and below WOT boost (~20psi) results in wide oscillations of boost between 10 and 20 psi, bucking, and eventual throttle plate slamming and limp mode.  Boost can be controlled at part throttle at around 5-6psi, but anything more results it erratic behavior.

N75 has been replaced with known good N75.  DV has been replaced with known good DV.  Turbo has been replaced.  Pressure tested for boost leaks.  N249 has been bypassed to verify it's not that.  Nothing as made a difference.

Logs of oscillations are included.  Screen shot of problem is also included.  If anyone has feedback it would be greatly appreciated.

Yeah, it's not even that Nye.  This is a simple stage I tune, stock turbo, stock just about everything...  as mentioned, even a stock tune exhibits the same issue.  It's something hardware, just unsure of what it is.

Was also replaced just replaced.

Yes, it has a MBC in there only for overboost protection.  It is run in parallel with the N75.  Needed so that WOT boost doesn't spike to the moon.  No amount of tuning will fix whatever is going on.

I agree with all of this.  MBC is no good, but until the root problem is solved, the MBC will stay in there for overboost.  The N75 is fine, tried other known working ones.  It very well could be plumbing related.  I am having my friend tap his boost gauge into the n75 lines to see if there seems to be a restriction there that is causing this mess.