Michael, I've also seen a trend for worse results after the monitor has been on for a while, vs. waiting just 30 minutes after it's turned on. However, I've run a lot of these calibration tests now and don't feel confident that anything like that is consistently reproducible for me. On average my results are now somewhat better than I reported in my last post. Can't really say why, though.
My best set of values, obtained 40 min after turning the monitor on, for my usual printing target 6000K 120cdm 200contrast were Avg=0.54, Max=0.82; and a few minutes later for Andrew's target ARsPhotoEditing values were Avg=0.56, Max=0.83. These values are not much higher than the two sets of values Andrew reported for his target, Avg=0.32, Max=0.57 and Avg=0.44, Max=0.72.
But most of the time my values for my target are in the range of Avg 0.60-0.95, Max 0.93-1.39.
I'm tentatively concluding that those values are good enough, given that dE values lower than 1.0-1.25 are said to be indistinguishable to the human eye, and I can't see any noticeable problems when I look at my images. So I think I'll stick with this monitor. But my mind is still open to other arguments.
P.S. For what it's worth, I also have a 3 year old, heavily used PA272W monitor, and its latest 2 sets of calibration values for my target are Avg=0.57, Max=0.96; and Avg=0.45, Max=0.76. Not much different than the values Andrew reported for his PA271Q.