Continued problems with no root cause or solution at present.
After a couple of additional short test prints (no more full size ones for the time being ) that repeated the problem I began to wonder if I may have a bad roll of 24" HM FAP, so I switched to a roll of Epson Premium Lustre that I'd used a few times on my Epson 4000. It's also thinner than the HM FAP.
Before I calibrated and profiled the Epson paper I used the troubleshooting guide to run through the HP solutions to fix banding (although this did not look like banding), they suggest a head clean, a head alignment, and a paper feed check. Unfortunately the head clean is done using the same special kind of wipes that come with the heads and are used when they are initially installed, so of corse mine had all been discarded and the printer comes with no spares. I reluctantly skipped the head clean step and ran the head align and paper feed check. No issues that I could see with the test prints made by those steps. So I tried the EPP Lustre.
No problems with the calibration or profile patterns (but I'd had none with the 24" HM FAP). A short test print looked OK, so I committed a 16x24....bad idea. This time I did not get the zebra stripe columns as I'm now officially calling them, but this time I got faint horizontal bands, pretty subtle as I didn't notice them until I got the print out and under some brighter lights In fact there appear to be 2 problems on this print, a repeating band about every inch or so, 1/8" wide and very slightly darker, and then a much more distinct dark band about 1 inch from the end of the print, this one about 1/4" but with less distinct edges.
I imagine I'll be talking to HP tomorrow.