New Audi Fan with history on 1988 Audi 90

["Walt Dwarnick" <walthome@ptdprolog.net>]

This is the first time that I am posting to this site.  It looks like a
great list and good source of information.

I own a 1988 Audi 90 in-line 5 cylinder that I bought new.  It just clocked
160,000 miles.   I love the car and hope to keep it going. 
I have solved a few problems along the way in my 12 years of ownership:
1.  Tapping noise - replaced lifters, but what really fixed it was
disasembling and tighting up the vacuum pump
2.  Electical switches AC, window, blower speed control, trunk light -
fixed them all by disassembly and filing the contacts.
3.  Clutch Master Cylinder - Had to replace it, and also replaced the slave
at the same time
4.  Rubber gromets on Brake Master Cylinder leaking - replaced them to fix
the fluid leak.
5.  Water leak at engine block to heater - replaced o-ring
6.  Oil leak - fixed by replacing the front oil seal
Did the regular maintenance plus struts, brakes, belts, and preventative
replacement of the water pump.

I am working to solve a problem now that has me searching for help.
I have had an idle problem for the past 2 years but it is intermittent and
I can't seem to pin down the cause.  It used to only occur during the
summer on hot days with the AC on.  But now it is starting to occur more
often and we are past summer.

The problem is that the car seem to go into a limp home mode where the
engine controls appers to ignore the ECU computer fuel control and the idle
drops to about 300 rpm.  When I shut the car off, to reset the system, and
then restart, the engine speed returns to normal.  

I don't think that it is the Idle Stabilization Valve. I tried resetting
the air flow potentiometer, which helped for a while, but the problem
returned.  When I pull off the connector to the air flow potentiometer, I
get this same idle drop effect.  Could be a short.  I plan to focus on the
air flow potentiometer, because pulling off the connector to the air flow
potentiometer really simulates my problem.  So I will have to definitely
eliminate a short as the cause.

I would appreciate any help with this problem and would like to hear from
others who have worked a the 5-cylinder Audi 90.

Regards,  Walt D

PS:  How long do these cars run anywat?  I hear that they go to about
250,000 to 270,000 miles before they need engine work.