VS 2008 - Threaded Ping - Click Another Site While Its Still Pinging Them It Doesn't Wait Before It Switches Site

Ive got an app that holds a list of site we have, At each site there are a few devices. The idea is when they click on a site all the details for the site are loaded and then a second thread sits and pings each device to see if its live. Im trying to do this in a seperate thread so if they click another site while its still pinging them it doesn't wait before it switches site. This all seems to be workling fine except if you click through the sites quickly, when you do that it will eventually come up with

Quote:

{"Collection was modified; enumeration operation might not execute."}

The error flags up on the "Next" line in "ThreadedPing"

Thread stuff

Public Sub ThreadedPing(ByVal dt As DataTable)
Try

Dim dr As DataRow

[CODE]...