Just a follow up on my problem. I decided to replace the waste-gate actuator and bought an actuator and boost controller. I've been waiting for warmer weather to install the new actuator.

In preparation for the installation, I wanted to check the travel of the existing actuator as compared to the replacement actuator. With the engine cold (for obvious reasons) I found the point where the actuator rod attaches to the waste-gate control arm and pushed it back just to check the total travel. The rod pushed back with mild resistance and pressurized the vacuum system. I could hear moans and groans from various actuators in the vacuum system. I cycled the boost actuator several times listening for any obvious hissing (leaks) but heard none.

Since pumping the boost actuator back and forth several times, the car has run near perfect for more than a month. The boost has not failed even after long distance runs on warm days and the transmission is shifting better than it has in over a year. Obviously, pumping the actuator knocked a piece of crud out of something somewhere in the vacuum system.

I still intend to change the boost actuator over to a pressure regulated type and remove the EGR value (to stop pumping crud through the engine), but in the short term...what can I say, the stupidly simple seems to work. I rank this fix right up there with the infamous GM "tap test" for their engine control computer (I used to own an 89 Corvette).