Way to go nice job.

I started by removing the fan and it's "tube" in the engine bay to disconnect the sensor connections (with Bentley help). Then under the car, removed the shock absorber lower bolt and the heater flexible hose to move them both aside to gain better access. Removing the sensor retaining bolts was awkward, compounded by what appeared to be some "lock tight" when originally installed. Had to be careful and patient doing this. Note do not remove the bracket that holds the sensors to the engine case! This took about two hours. Installation was also about two hours.

There are a number of good threads on this. Plug this into your Google and settle in for some good reads:
sensor replace site:Porsche 911 Technical Forum - Pelican Parts Technical BBS

Congrats on solving the problem. I was going to suggest the sensors too, as I ran into that same problem on a friend's car ('78 Targa with a 3.2 transplant) that had intermittent stalling problems.

I would suggest others reading this should consider removing the mounting bracket for the sensors. I know doing so disturbs the clearance/air gap, but setting the clearance is very easy. Reason I say it's a good idea to remove the mounting bracket is because often times one of the sensors gets stuck in the sleeve that carries the sensor. On a few occasions i've had to destroy the old sensor to remove it, along with spending some time fixing up the sleeve so it's not so mangled from removal.

Did you get the Porsche part number sensors or the BMW equivalent?

I checked various sources for BMW part, but no one had them in stock. I bought the cheaper brand our host sells.
One sensor was stubborn to pull out, but turning it back and forth plus patience got it out.