My 1991 960 was hunting and staling and the fuel economy was getting worse.
Error codes 2-3-1, 2-3-2 & 2-3-3 were registering quickly after clearing. They are all to do with idleing.

I listed lots of relevent advice on Brickboard. Cleaning the sooty carbon deposit from out of the Idleing bypass valve with acetone was enough to stop the Hunting and the errors stopped immediatly.

Error code 2-1-2, indicating a faulty H2OS (Lamda) sensor was still occuring.
As advised on this site measuring the sensor voltage output with a volt meter (A Fluke) showed a slowly changing voltage over minutes as the exhaust warmed up, it should switch levels more definatly.

After changing the one sensor, at the input to the catalyser, the voltage switched every few seconds between approx. 0.3 &.8 volts.

This seemed to show that the exhaust gas temperature was changing in response to the sensor information to the engine management system.

After a few hundres miles the fuel economy has dramaticaly improved even when towing a load.

The car had done 160,000 miles. It has a 1.8 Motronix system. The top of the engine looks very clean and I could find no air leaks.