Wondering if there is a bug in Word 2007 and 2010.

Not all the time, but periodically, when I either save a document or autosave runs, I am no longer able to "undo" anything I changed before the save/autosave. Needless to say, this is undesirable when one is writing and changing large documents such as a dissertation! It happened periodically when I was running Word 2007, and now again when I've just installed Word 2010.

I saw in another forum that someone had Google desktop and that this was causing the problem for them. I don't have Google desktop, so that can't be it for me. I am running Vista Home Basic.

Has anyone encountered this, or heard of a fix?