Lets hope you get this other turbo to use off your mate and see whats what because it can't just work then not work. The turbo has no play in it,spins freely and has no damage to it to stop it working how could it just stop providing boos instantly doesn't make sense because even a fucked turbo will not just stop unless the shaft snaps or something. Like I said how are you acheiving this higher boost level of more than 8psi??? Your problem could be amal related or actuator related r bleed valve if everything else is set correctly.