91' FRST

At 16psi as it reaches the 6200-6300rpm, the car slows down, rpm falls down to 5000,
and the pressute too. It exists until I step off the gaspedal.
If the pressure is higher, this symptom appears sooner:
18psi-> 6000rpm
20psi-> 5500rpm

Why does it happen?
What can I do, because i wanna use my car on 20psi?

spec:
1.6 low comp Mahle
superchips(unlimited boost)
FMIC
denso racing fuel pump
305ccm injectors
champion QC59C
T3 stage3



thanks for your help