When Performance Monitor loses the connection to a monitored item, it does not close any open view for that item. Later, if the connection is restored, trying to open a view of the item fails. Instead, Performance Monitor brings the existing inactive view of the item to the front.
Workaround: Close the existing inactive view, and then open a new view.
You can use the Close Inactive Views command on the Window menu to close all inactive views without having to locate and select each one.