No boost when upshift done above 5000rpm

[arty288]

I have a Legacy GT JDM MY1997

When I change up a gear whilst engine is in single turbo charging range (also on preparatory 2ndary turbo range), the engine responds fine and boosts as normal.

When driving hard and revving past 5000 rpm - on the next gear the engine will flat out refuse to accelerate, or rather the revs creep up extremely slowly and there is definitely NO boost.

note: if i gun it and rev it in any gear it will happy boost all the way through the entire rev range, but can't stay in one gear all the way unfortunately.

I have no idea what could be causing this as my understanding of my engine is pretty basic, could it be a sticky Exhaust relief valve, dodgy wastegate.