Can’t download mail from Gmail.

I am using Windows XP in my office. I have many works in mail corresponding. For my mail corresponding I always use Gmail. I open Gmail in Mozilla Firefox software. I feel comfortable to use Mozilla, but occasionally my boss tell me to see previous mail, thus I need to open Mozilla and go to internet, so by one friend help I configure outlook express in my computer, my friend show me how to configure it. Ok it works. But sometimes it cannot send and receive mail. Error occurs. My internet ok, but error occur, but after few minutes or hours it works correctly. So what’s the problem?
