Contact your host, there are many things that could be going on that only they could identify. If you get hit by multiple search engines during a busy day you could have memory faults or exceed your concurrent connection limit in apache.
With a board as big and popular as yours I would learn how to use the command line. Peepcode has some good screencasts to get you started.
http://peepcode.com/products/meet-the-command-lineA few command that can be very helpful in identify problems are:
free -m : will tell you how much memory have are using and how much is available, might depend on your environment a bit if you are using a PS or not.
ps aux | grep http | grep -v "\(root\|grep\)" | wc -l : tells you how many concurrent apache connections there are.
