One other thought which is easy enough to test.

I can stall my '87 3.2l or find it running a little lumpy:

1. usually first thing in the morning;
2. when the engine is cold (i.e., ambient); and
3. I have fueled with 10% ethanol/gas.

Once the car warms up for a few minutes, no problem. When I fuel with pure gasoline, never a problem.

 

Quick update after I returned today. I could not diagnose the problem any further. We have now around 70 degrees and it simply would not show the problem.
I just started looking into the vacuum hoses and found that the big hose connecting the oil tank/throttle valve housing elbow and a small one holding the vacuum limiter warrant replacement. Don't think they explain the problem but they look like they will go sometime soon. I will carefully check the other hoses too with some more time at hand.
Concerning the idea with gas. Having the DME chip, I need to fill up with 98ROZ gas (don't now the equivalent in the States, at least Premium gas) and that has lower ethanol content. And I always fill up at the same gas station. Amazing what good mileage that thing has. My GTO plays in a different league
I will report back when the problem returns and I don't find the issue.

 

Took the 911 today to commute. 50 degrees and higher humidity. Ran fine for the first 5 minutes without any stumbling or issues. Then stumbling would appear, most pronounced at 1800 rpms. Pretty annoying, even if I can avoid that small rpm range most of the time. Stayed there for 5 - 10 minutes and then disappeared. Guess this will be hard to find. As said, I will start with replacing those two hoses. Nearby shop (see other post) said they would look into ignition wiring.

 

Might help while you have the car in the shop if they can measure the CO or A/F ratio from the exhaust to determine if you really are running too lean/rich.