Mail delayed in fetching mail from IMAP server

My Mail application (Leopard) is sometimes incredibly delayed in downloading mail from the server. If I have Thunderbird open on the side sometimes I receive mail in it but have to wait 15 minutes before Mail gets it. Also, my iPhone gets the mail in time always, though one would think that the mail fetching system is similar to that in Mail. Does anybody else have this problem and know a way around it? It's really frustrating and I'm considering moving to Thunderbird for good.

 

Yes, it's set to "Every minute", but as I wrote above it sometimes takes much longer (I think it has in rare cases been hours) for it to download messages.

Any other thoughts anyone?

Thanks!