This is the second RB25 i have ever seen do this. We have tried everything aswel, even new HKS turbo, still creeps!
1st thing would be to get a aftermarket ECU so you dont have the fuel cut happening... and new AFM.
Z32 & FC would be fine.
Try disconecting the wastegate arm off the flap and leave it wide open, 1 would think that it would make no more than 7psi at 7,000rpm. We also tried this and got 15psi???
Creep starts to happen always at around 4,500 - 5,500 rpm, and keeps making boost like a super charger.
I would like to know the answer to you question aswel.