'87 300D (603) Low Boost Pressure (9 psi max)

[SW]

My car runs great but the turbo seems to be MIA these days. There is a noticeable lack of boost from 45-65 mph. I installed a boost gage between the switchover valve and the intake manifold boost line. In 3rd gear under load @ 4000 rpm, the max boost that the gage is reading is 9 psi. From a standstill, the boost will go to 5 psi quickly but afterwards it's a dog until the rpms build up. I'm thinking the wastegate is stuck open or out of adjustment. How is the wastegate adjusted on this particular model? Is the procedure similar to that in the w123 turbo's? Any replies would be greatly appreciated.