'90 200 - various minor problems

[tubman <tubman@alaska.net>]

I have a variety of problems with my 1990 200 Quattro.  All
are intermittent, so they've been tough to track down.  I'd
appreciate any suggestions.

1) The "check engine" warning light comes on when
accelerating with RPM's between 3000 and 3250.  The light is
only on when the engine is in this range.  It seems to
happen more when the outside temperature is cold.  More load
makes it easier to occur.  I can do get the light to come on
consistently in 5th gear.  The lower the gear, the less
likely the light will come on.  I'm guessing this is the O2
sensor, but I'd like to be more sure before replacing it.

2) The temperature gauge sometimes is slow to move from
cold.  Then it comes up quickly to read normally.  This has
happened since I bought the car.  Recently, the overheating
warning light comes on occasionally.  The car shows no signs
of overheating (e.g. no steam, temperature gauge does not
read high temp).  If I turn off the engine and immediately
restart, the warning light will be off, and does not come
back on.  Also, the warning light sometimes goes off even if
I keep driving.  (No, I don't drive this way a lot, but it
sometimes goes out within a minute.)  I think this might be
a bad sensor or electrical connection.  Which sensors go
with the gauge and warning light?

3) When I turn on the headlights (or just the parking
lights), both turn signal indicators and the high beam
indicator come on.  The lights between these indicators
become dim.  If I turn on the signal indicator, the proper
arrow and the high-beam indicator will flash alternately
(varying intensity, they never go out all the way.)  Is this
just a bad switch?  How hard is it to replace?

Thanks,
Ken Tubman
ktubman@alum.mit.edu