OK, I'm at wit's end. I've validated my XML sitemap using 4 online tools-- even the W3C validator. I got a clean pass on all of them.
I saw where Google didn't always like the <lastmod> tags, so I removed those. No effect. But they accepted the <lastmod> tags in the other sitemap.
All the &'s are escaped using entity codes. (Yep, screwed that up once before when carelessly saving into ASCII out of Notepad). The sitemap is where it belongs and where I told G to find it. But resubmits don't help.
This sitemap uses the same coding that a sister domain uses. Google has no problem with it, but they don't like my new one.
Does anybody know of any gotchas that I may have overlooked?
Thanks for everybody's help. It turned out that the sitemap was perfect all along.
This site was just moved from one hosting company to a new one. A third part IT guy controlled the client's domain account and wouldn't allow me access to the DNS settings. He screwed up the DNS somehow and, although the site was live, Google wouldn't accept the sitemap. Nor could they see my pages when I "fetched as Googlebot" in G Webmaster Tools.
He corrected whatever he did. Now the site rankings are back up as before in Google.
I still can't figure out what he did that would allow the site to be live on the new nameservers but Google couldn't/wouldn't spider the pages.