I'm using a Linksys WRT54G router, and have two laptops. One is a Toshiba running Windows XP, and the other is an Acer running Windows 7. When I have only one computer connected to my wireless network, the connection runs as expected - perfect speed and ping times of only 55-75 milliseconds to both google.com and download.com
These results are the same both when using the PING function from a cmd prompt and when using PingPlotter. The problem comes when I allow a second laptop to connect to my wireless network. Within 3-5 minutes of connecting the second computer to the network, my PING times jump to over 1,000 milliseconds. I've had spikes where the results were as high as 2,500 milliseconds to the same server. After disconnecting my second laptop from the network the problem resolves itself.
It doesn't matter which computer connects first or second - and I've used other computers just to test to see if it was a configuration problem on one of my laptops - the results were the same. Anytime a second computer is connected to my network, but PING times because unbearable.
I've ordered a couple of new routers, but it'll be at least a week before they arrive and I'm going bonkers!