[ischmitz]

The Hall sensor fails "silently". It doesn't even through a CEL because its failure is not degrading emission. But you will be slightly down on power because the ECU permanently retards the ignition by 6 degrees.

The reference sensor can do what you are describing. It can fail intermittently. The other things to look at are the O2 sensor connector and the connector to the AFM. There is a service bulletin from Porsche about placing a Zip tie onto the cable existing the AFM connector. The cable touches the firewall when the engine moves under load and that has caused running issues.

The three-pin O2 sensor connector is sometimes spotty. It makes the O2 signal momentarily drop to zero and the mixture control drives all the way to fully rich where it becomes incombustible. I had this issue and the engine would cut out under cruise conditions like "someone pulled the plug", Then all of a sudden it would kick in again.

A simple test is to unplug and re-plug it a couple of times. This cleans the contacts. In my case it gets things back to normal for about 6 month.