Nov 15, 2009 12:45 PM|RickNZ|LINK
To get Fiddler to look at your dev server, just replace "localhost" in the URL with "ipv4.fiddler" -- leave the port number as-is.
I don't understand why (or exactly when) you're seeing a Master Page. Do you mean as part of the same response with the .ashx file? I can't imagine how that's even possible.
Can you tell anything from the Fiddler trace?