2.5 Turbo Boost problem.

[DieselDummy]

Hi,
I am a decent machanic but a diesel dummy as in my log-in name.
I have a 1991 300d 2.5 turbo that is having intermintant boost issues that seem to be and electronic based problem. I installed a boost gauge so I can track whats going on and half, some, most, or none of the time my car will boost and when it's not, I can get it to attain full boost just my activating the Accl. lever of the cruise control. Once it starts to boost it runs great and may go 1 min./day/week working correctly before the boost drops out again.
Whatever electrical device the Cruise Control circut is controling must be getting or not getting a signel from whatever is supposed to control it. What controls the boost system, is it the CPU or something else? What can I look at ?
thanks in advance!
kevin