Mail has started playing up...please help!

Just recently mail has really been acting strangely, I have had my PB since November and this has just started in the past week. Some of the problems:

1. It will check my gmail account for email, but on my .mac account all I get is the circular progress icon and it is not retreving any email from it.
2. When mail is already open, if I click on it in the dock it will open up, and there will be no mail visible in any of the folders, even if the icon in the dock is showing there are new messages.
3. It NEVER shuts down, I always have to do a force quit. If I shut down the computer with it open, it always cancels log out and I have to force quit it manually. Sometimes what happens if I try and quit it normally, the window will disappear, but in the dock the little black arrow underneath it shows it is still open. If I click on the icon it does not let the window open again, but it still remains open and has to be force quitted.

Does anyone have any ideas, I am really at my wits end with it!