You are experiencing a drop in boost in third upwards map5 correct?
Try map2 (switch using the steering wheel button method) and does that feel harder?
You may be losinbg pressure - leaks from wastegates etc.
A small leak will be managed out but the snail(s) are working overtime to keep the pressure on target.
This will show as rapid heating and retardation on an autotuning map like map5.
Older car, maybe higher mileage? Perhaps the inlet is coked up too, that wont help.
Are you using a lot of oil? Oil in the mix wont help.
As above you really need to datalog everything.
But unfortunately one common thing that will lose you output is the dreaded HPFP - even before it raises a code or poor starting from cold.