Check for the infamous broken cotter pin on the wastegate arm. Also you might have some serious crack around the wastegate itself.
Might be a simple as a broken vacuum hose going to the wastegate actuator or a boost leak some where in the IC pipes.

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'91 Laser RS Turbo-"Sound Advice" guy.
www.geocities.com/motorcity/speedway/5051
Turbo + Intercooler = The replacement for Displacement

'06 Magnum R/T (Yeah -it's got a HEMI!) Mods beginning!