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85' 5KCST Check Engine Light

Hello everyone!  I haven't ever posted to the Quattro List before, but 
the wealth of information here is impressive to say the least!  I 
recently purchased a 1985 5000CS Turbo (auto) with 92K miles.  It is in 
excellent condition with one problem:

The Check Engine (yellow engine block with lightning bolt thru it) 
illuminates at highway speeds when the engine rpm goes past 3000 and the 
boost rises past 0.7 Bar.  In other words, at about 65 MPH (when the 
cruise is on and it cracks the throttle to maintain the speed on a hill, 
for example).  Accelerating from a stop, the light does <not> come on 
and the boost gauge registers normally (i.e., 1.5-1.6 bar) under heavy 
accelleration, but the light will sometimes blink briefly on upshifts.  
The closer you get to highway speeds, the more easily (i.e., with less 
throttle) and frequently the light comes on, until at 65-70 mph it is 
virtually impossible to open the throttle more than a crack without the 
light flashing as the boost hits 0.8.  However, if you are "coasting" 
(0.3-0.4 bar) at this speed and then "punch it" (full throttle), the 
boost builds rapidly  up to about 1.4 bar, and then the light comes on 
and stays on until you back off. 

When the light illuminates at highway speeds as mentioned above, I can 
feel a drop in power and I have been reluctant to drive the car 
aggressively out of fear of damaging the engine.  

I have run several tanks of 93-octane through the car, with no change. 
Other than this, the engine runs perfectly.

If anyone has experienced a problem like this or has any input, I'd sure 
like to hear your comments.  

Thanks in advance,