This is a largely undocumented problem with GMail over pop.
Read this article. Gordon's Tech: Gmail’s undocumented POP3 download limit
It appears that your download limit is lower than that stated (340) but I believe it is the same thing.
I think the only way yo fix this would be to setup an IMAP GMail in Mail.
Are you using normal GMail or google Apps Gmail?
If you need any help doing this just ask, I'm here to help.
P.S. This forum is generally reserved for questions about Windows Seven or involving Windows Seven, if you have Mac only problems, we generally recommend you use Apple - Support - Discussions - Forum Home
instead because there will be many more people who know their way around Macs unlike here (where it's just me