Hi John,
The feed manager cache doesn't keep anything permanently so if it expires or is cleared it will be as if its all new again.
The easiest solution since you are talking about an internal feed from the blog is to edit the blog post and uncheck the box for "Include In Feed" if you don't want that post included. Or if it sjust older items you don't want in feed manager put a lower number in the Feed Manager setting for "Show posts for up to this many days old".
Also you can set a longer time for the feed manager cache to last.
Hope that helps,
Joe