Having trouble syncing yahoo account to Mail and iCal

I have over 27000 emails on my yahoo mail server, but I only show 9900 or so on Mail, so something is not syncing. I discovered this when I was purging a lot of email, but when I deleted these emails in Mail, 20 minutes later, they downloaded again from the yahoo server. Also, I get the triangle with an exclamation point next to the account in Mail and when I use Connection Doctor, it says that it is unable to login. Please help.