Intermittent low boost - most likely causes?

[Benjamin Weste Pearre]

Hi all!

When my 1990 200tqa makes 1.4 bar, it's wonderful.  When it makes 1.1
or 1.2 bar, it's a slug.  It goes between these two states every so
often, but not fast --- often I go for a drive and the car either can
or can't make full boost for the duration of the drive.

My question is: what are the most likely causes for such a problem?
Intermittent problems are of course difficult to diagnose, so I guess
I'm stuck replacing one thing at a time until I stop seeing the low
boost.  What should I replace first?

Things that may possibly be related:

* No fault codes.

* Maybe possibly it's somewhat correlated to temperature --- on cold
  days it's less likely to make full boost until the car has warmed up
  for a while.  But I am not absolutely sure --- sometimes it keeps
  not making 1.4 after warming up, or on hot days, and sometimes it
  makes full boost for a while and then decides not to anymore.

* When I first got to Boulder after driving from Boston, the car's CO
  ratio was at .12, which meant I got fantastic mileage, but little
  power, and the car stalled until it was really warmed up.  The
  mechanic "adjusted it to .8" which solved the symptoms, but does
  this indicate that a sensor (O2?) is faulty?  The O2 fault code was
  lit when I got the car back, which suggests that he at least
  unplugged the O2 sensor, I think.  I'd assume he tested it.

* I'd been running on 89 octane fuel for a while (when living around
  Boston I'd rarely needed to floor it and hadn't noticed that my car
  was a slug).  My mechanic pointed out the error of my ways, so now I
  run 91, which is the best Colorado has to offer.  Is there any way
  to tell whether the ecu is still detecting knock?

* Sometimes in the cold the engine stumbles for maybe 20 seconds at
  really low RPM (like 200 or so) when I first start the car.

* The throttle switch checks out fine, but I understand that it can
  fail intermittently --- should I hook up a multimeter and go for a
  drive?  It was "repaired" 8 years and 80000 miles ago; I might just
  be due for another anyway?

Thanks very much for any suggestions of what to replace first!

-Ben

--
Ben Pearre          1990 200TQA          http://hebb.mit.edu/~ben