the problem is the boost controller is set at 12 psi (aem tru boost controller). im suing an sts turbo on a 6.0 with a t56. the boost fluctuates and doesnt stay on 12 but reaches it in some runs and other runs gets 7-8 only in 3+ gear. 1st and 2nd gets to 5-6 psi. i dont know if the problem is from the controller or the wastegate.

 

the turbo maybe to small....

i'm not a turbo-guru i am still learning but maybe someone else will chime in

 

the turbo is smaller than what you would have behind a 6.0 but i have it since it helps boost in lower rpms. its a 60-1

 

do you have stock manifolds or long tube headers?

 

check the diameter of the vacuum lines to the wastegate -- too large of a line will act like that.