Trouble with WOT Boost control

[A//// Guy]

1991 GSX with 20G and Tial BOV and Tial 38mm external wastegate off the O2 housing, and a Joe P MBC.

Every time I get the car up to the 15PSI spring pressure of the external wastegate the car immediatly starts stuttering the boost up and down (goes to 15psi then stutters downward) The BOV is basically going nuts... If I dont go up to 15PSI everything works fine (bov and boost).

I dont think I have it setup correctly with the MBC or maybe I need to use 2 different hose sources instead of T-ing off both the BOV and wastegate on the same line?

Here is a diagram of how I have it connected now - The MBC shouldnt be controlling the boost because I have it set to the lowest it can go, and the Wastegate's spring is 15PSI- so that is what the cutoff should be.

Let me know what to do or how to hook it up, maybe my MBC is going bad or something....

TIA
Peter