The turbo underboost condition can also be the hose- check the hose connection at the intercooler - they are known to pop off or leak a little at the connection. It is somewhat obstructed to see so look from the bottom as well. Second cause is the boost solonoid under the intake - check the small rubber hose to it they will get brittle and split at the nipple connection...then check the solonoid itself. Only then would I look at the turbo being the issue. If so, it is about an 1 hr to 1,5 hr job and replace the oil seals including drain back tube seal.