Apple mail downloading old messages again

This happens to me every once in two/three months and is annoying like **** but have never been able to find situation. Here is what happens, i have 4 emails configured on my apple mail and they work just fine. I have settings where it leaves the copy of email at the server.

Now, after every few weeks it starts downloading some old emails automatically and it doesn't download all emails.. It just pick and chooses. For example, friday i had 50 unread emails and saturday evening i had 285. When i started checking my email, these are mostly old emails that i have already read but they are on server.

The oldest i got is from 12/17/2012 today is 3/17/2013 So its last 4 months but i also know its not downloading all emails in this time because then it would be like 10000 unread emails.

I can delete the ones i got again and kinda fix it temporarily but this will happen for sure again in couple of months. Any solid way to fix it forever? I care now because i only have 1 GB Storage left... lol, i know its lazy

Thanks a ton in advance.