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Inconsistant boost level

Would anyone have any suggestions on where to look to determine why 
the boost control on my 90 200 is very inconsistant? Sometimes, the 
boost goes right up to 1.4 bar without problem, and other, it doesn't 
go above 1.1.  The interval between Ok and not ok can be very short:
as in its fine, and then two minutes later, bzzt, no boost.

The load, throttle position, rpm  and gear are all about the same when I  
have checked. I have verified the inconsistancy checking the boost 
according to the manual (35 to 55 3rd gear). 

So far I have tightened all the ic hose clamps, raplaced one faulty 
temp sensor and one maybe faulty temp sensor (one sends the engine
temp signal to the computer, the other sends the over temp signal),
replaced the air cleaner (previous owner never changed it as faras I could
tell) and repaleced the crankcase vent hose. All of that improved the 
performance markedly when the boost control was working, but that is not
all the time. The car allways runs fine no matter what the boost.

I am inclined to check out the frequency valve to see if it is sticking.
Any other ideas? 


Dave Michael

90 200 qw
87 alfa milano 3.0