Pull off the intake, stick your fingers inside turbo inlet and feel for vertical movement. Take ur time, it is a little bit of an awkward position. I got me a new turbo based on the play that I, and a gm service tech felt. Mine wasn't holding boost like it should either. If I gave it bout 1/4 throttle I could watch boost go to 5 and fall down to 1 is a short period of time with the same throttle input being applied the whole time. Now with new turbo it stays steady and doesn't fall off. This is with GMTU and trifecta. It holds

Only other possibility I can think of is belt slip which usually only happens when you let off clutch after shifting. Race shifting..

 

Thanks man. I take a look for vertical play over the weekend and let ya know what, if anything, I find.