Why does my web site keep going down and then coming back up?

I have a web site that I monitor with pingability.com Some days my reports are fine and some days I will have an alert that my site did not respond with it 30 secs. I’m looking for a way to determine what is happening on my site when it is slow to respond. I have it pinged every hour. What things could I look for in my web stats logs to try to explain why the alerts keep happening? I’m running it on a windows based server.