Page Corruption: Visual Editor and Page Revisions vanish when clicking Update!

I am trying to migrate my current web site content into the Atahualpa Theme 3.4.5 in WP 2.9.1. I have tried copying the content both from the site front-end, and also from the source code. In both cases, I'm copying tables with a mixture of text and images. All goes fine for a couple of hours of reformatting (other than font spacing not alway sticking!), and I save regularly. Then, for no reason I can discern, one of the times I click Update, I see the HTML code on the Visual Editor tab and my list of Page Revisions vanishes! This has happened to me twice now, the first time I had copied the content from the front end, put in the images fresh from my Media gallery, published the page then continued editing. The second time I took the source code for the page and put it onto a new WP page, and proceeded to edit, add images, etc. Either way, same result. Esmi on the WP forum suggested I try restoring a default theme, and sure enough, the page normalizes. So this is an Atahualpa issue. Is there a table bug? I don't see an easy way to create tables in visual editor, and I'd rather not do everything in hand-coded HTML. I can't find anything in the forum quite like this combination of issues, and I'm supposed to migrate my whole site this weekend. Can anyone help?? I set up a much larger site on Atahualpa with no problem, including copying content, but it wasn't tables or so many images. Can these pages be fixed? Or do I have to find a different theme? I do not have any plugins active on this particular site. I saw the info about some "concatenate" code, but was not sure if that still applies to 2.9.1?