We've had a sitemap up for years, only showing the newest content, no
problem. But, I just added more links to it.

Something screwy going on. I'm moving between page 1 and page 2 every
couple hours today. And, the Google result counts different each
time, 1 - 469,000,000 and then 1 - 505,000,000 for the same term.
Depending on record count, I'm on different page.

Kinda like the request is being served by different servers at Google,
maybe server farm, and they don't have the same index (similar, but
not the same). Just a thought.