I had a WP 2.7.1 Blog this first occured. Because I know that there were bugs in the 2.7.1 Feed system I waited until 2.8 came out and installed it right away. It did not change a thing.
This Behaviour is quite esoteric. I am a bit puzzeled on how to find out what it causes it. I really need help and input from third parties because I do not see clear any longer.
I have a Blog with a static frontpage.
All pages but the static frontpage _do_ work after rebuilding permalinks. After some hours, let's say 8 hours but everytime after 24 hours, alle pages but the frontpage do not return their content. They get converted into the standard feed of the page.
* I deactivated one plugin after the other - no change.
* I changed to the default theme but the feed was still delivired.
* I hacked WP Core sothat is_feed() always returned false. Then not the Feed but the standard lineup of posts was returned on the errorneous pages.
I do not know what is causing this. There is a secondary WP_CRON method I might try as well. I will check that out.