FF makes my Comp RestartFollow

I dont know why, But about 5 min after i log into Final Fantesy 11, my computer restarts. It happens every time. I dont know why, or have any logical explanation, it just randomly restarts. I think it has somthing to do with the memory stick, but it could be the graphics card or RAM. It worse in crouds of people, and doesnt kick me off for like 7 min when no one else is around. I have all the graphics and everythng set to Minimum and nothing seems to help. Then when i boot up my computer, it says somthing about a critical error. Can anyone help me? Its making it hard to do anything.