Can someone help me make sense of this?

[quattrobros]

So ive got a 3b swapped 90 and the previous owner had a TAP chipped ECU and an upgraded wastegate spring.

With the n75 unplugged i get 12psi and with the n75 plugged in i get 16psi surging between 12-16 psi.
Now ive got my hands on a stock wastegate spring and i get 6psi with the n75 unplugged and 10psi with the n75 plugged in no surge.

I dont understand what the "tune" is supposed to do and why it surges.