Lack of Boost, '88 5ktq

["Mike Hopton" <mike@genesis-microchip.on.ca>]

     The patient is a 1988 5ktq. I've been fighting a lack of boost problem 
     for some time and would like to seek the list's advice before 
     proceeding.
     
     So far, I've replaced most of the vacuum lines, removed and inspected 
     all the large diameter hoses from turbo to intercooler throttle body 
     to intercooler etc, replaced the WOT switch. I have adjusted the DC to 
     38% and it trips to 70% on WOT, car idles perfectly (touch wood). Pull 
     the dip stick the car hesitated but keeps running, remove the oil 
     filler cap she dies. Crank case ventilation hose and other breather 
     hoses all check out okay. Never seen more than 1.2 on the digital 
     boost gauge and the Autometer has never read more than 4psi boost. I 
     inspected the cold side of the turbo, the blades felt fine and rotated 
     freely with no perceived play.
     
     I'm thinking the lack of boost is on the exhaust side and I'm guessing 
     either the hot side of the turbo or a blockage in the exhaust system 
     (cat?). What would be the best way to proceed to try and isolate the 
     problem? 
     
     Thanks and regards, Mike