idle problems

[Mark Zvolanek <mzvolane@sydney.ipex.com.au>]

Gday q-listers
Please help me with this one. I am new to Audi I had this one for few   
months now.
I am still having idle problem on my '91 Audi 90. So far I have cleaned   
ISV, replaced crankcase breather hose, checked idle switch.
When the car is stationary it idles well, when I pull out oil dipstick it   
recovers in a second. When I am in traffic not going more that 40km/h or   
so and when I stop it still idles well, i.e drops down to 800RPM and   
never below. But as soon as I go faster (approx more that 50km/h) and   
then stop idle falls down to ~600RPM and then it slowly moves up to   
800RPM or it occasionaly stalls. Even if I try to assist it by depressing   
gas pedal slightly does not help, it seems to wants to get up to 800RMP   
by itself.
Only thing I have not changed yet is oxygen sensor. Would OXS be causing   
this? When I bought the car all was well. Would OXS fail from one day to   
next, or does it fail slowly? Any recomendation on which one to get, OEM   
or universal.
Where would be the most obvious place to check for vacuum leaks. Could it   
be a air leak that occurs only when engine is under load?
Someone recommended me to check tps malfunction, what is tps?
Thanks for reading up to here.
Any help or comments would be greatly appreciated.
MarkZ