Turbocharged 98 LS

if that doesn't do it--
chances are your ECU is on closed loop-

I'd hghly suggest you get a missing link. its liek $60. it ****s with your ECU's mind, so it makes it think its not seeing boost... and turns it off closed loop.

as for your idle- did you mess with you idel control? throttle cabel length? or like above, perhpas you swapped sensors...

And i can't belive you don't have a BOV.

I hoipe you dont care about your turbo.
99% of turbo failure is caused by reverse airflow... when you let off the gas, the turbo is still spinning, and that air needs to get released-- otherwsie, it goes backwards into the turbo, and evetually, you get 0 psi :(
bad boost... need BOV