Thundebird: problem with receiving messages

It used to be fine but is not any more.

While there are surely new messages on a server (Gmail in particular) Thunderbird doesn't get them. There's sometimes a message at the bottotm : receiving messages (i.e.) 1 of 5 but after a while it stops and no new messages are loaded. I'm sure everything is well configured as it used to work fine until these days.
Is it b/c there are too many messages on my account??

What's stranger is that sometimes TBird's trying to get 'new' messages which've already been downloaded before. It displays then 'receiving messages 1 of 21' (while only two or three are new!!, not 21!) and sometimes one message is downloaded 30 times at a time!!!

Is there anybody using TBird with the same problem??
Or sb who's got an idea how to solve it??

I'd appreciate it as this problem's extremely annoying....
(even more than a shouting wife behind your back )...

And by the way,
why is TBird so slow??

 

I should've stated it before:
I've got two mail accounts on my TBird, both of them are Gmail - one like private and one kinda bussineslike... The second one works OK, it receives all messages from the server.
But the private one doesn't. I got some mails on TBird recently which have already been on a server for a week!! Why is that?? Shall I use two different mail-clients for two differentaccounts?? Or is there a way to fix it??

 

Suggest going here for help:
http://www.mozilla.org/support/