do you think it could be valley of death ? and your not giving it enough stick to get on the secondary turbo , think its broken so back off ?
maybe fit a boost gauge and observe?
or it could be failing fuelpump
look at a dynograph and see what i mean about valley of death
it has to relaese most of the boost the primary turbo is making so it doesnt stall the secondary then they both bring the pressure back up
the boost graph for that woud look like 10-12 then falling to 4 at the big dip on the graph then back up to 12-14 ish