First off. Are you trying to test your speed to a certain website, because that isn't a good way to do it? You understand that ping is used just to make sure that the machine on the other end is there.
As for why the ping time is going up and down, that is most likely do to the latency or the minimum amount of time that it takes for the very small ping packet to go from your computer to the host, then back again. There are many reasons for this and I am not going to list them all. But some of the most common reasons are: Firewalls, network card speeds, and modem speeds.
Here is a website that explains Ping in more detail: http://www.numion.com/faq/ping.html
I hope that this helps a little bit.