Help! Keeps downloading ENTIRE email inbox

Hi all,

I've had a problem with my Touch Diamond for a while and was hoping someone could help me with this.  I have my Yahoo email set up on it and I have it set to automatically check for email every 5 mins, but to only download emails from the last 3 days.  Usually this works correctly but about once every week, the phone will sit there and download my entire inbox of thousands of emails.  Then the phone obviously slows down enormously and I basically can't use it until I tell it to clear the inbox (which takes around 30 mins).  It then will work correctly for a few days and then do the same thing.

Any help would be suuuuper appreciated.  Thanks!