I did a search and it seems that there still is no answer for this, so I'm bumping it up to the top to see if anybody wants to take a stab at it.
It seems that my emails are randomly disappearing from my Storm. This also seems to be not just a Storm issue, but happens on the Curve and Pearl from what I've read on the search results.
These are not emails that are 30 days old, these are emails that are less than 24 hours old!
Memory issue just got thrown out the window. Have you tried deleteing the account and then readding it. I saw another person that said they upgraded and that they have had the issue since that. Maybe give that a try