so ive been chasing this problem for about a month and a half now.... i have a d16y8 with a garret t3 .42/.48 (not very large at all).
I was tuned on 12 lbs about 2 months ago and everything was running great. the car held 12lb consistently throughout 3rd and 4th gear perfectly.
then, for some reason one day, i saw the boost fall of hard in 2nd gear from 12- 7psi. and then in 3rd from 12psi-9 psi. i falls off at about 5800-6k rpm. now its making 12psi in 2nd and falling to 7psi still, making 13psi in 3rd and falling to 9psi, and in 4th making 14psi. i havent gone to the top of 4th to see if it falls off but judging by the pattern, it probably does.
Im using a tial wastegate and a hallman boost controller. Ive gotten the "your wastegate is stick open" statement about 10 times, Ive tried 3 wastegates and 3 boost controllers, none of these fixed the issue. the only thing i changed since i was tuned is put an oil restrictor on the turbo. is it possible that maybe im not getting enough lubrication in the turbo under high boost or could it possibly be something in the tune with the duty cycle thats affecting the load on the motor??

 

do you have any shaft play in your turbo? sounds like its dying.

 

i had a similar problem with my turbonetics t04e...when im in 2nd and have full boost then switch to 3rd id get no boost at all...then shifting to 4th id get boost again...i was clueless until i pulled the filter off the turbo and realized that my compressor wheel had shaft play...so i sent it out to Turbo Auto in Nashville TN and Jeff (owner) rebuilt it for me for $120...Jeff told me it was fried to not enough oil feeding the turbo...so i drilled my restrictor to 1/16 inch and now everything working fine