Could be also the turbo is shagged, mine went ever so quick, one minute perfect then 20 miles later 2/3psi boost!!
I had a look at turbo and the turbine housing was shagged, so much so that I needed a new one so put in a new core with Comp housing machined out to T34(large turbo escy), it is the same size internally the escy one looks much bigger but restricts down internally.
Add a 330 bearing and step gap seal for good measure.
tabetha