Well here is how you can rule out dns via tcp issues. If you get into a dos prompt and ping 4.2.2.1 you can make sure you have connectivity. If you telnet to port 53 of 4.2.2.1 you can make sure your machine can do dns lookups. If you are connecting on and off have you looked in the event viewer to see if there are IP conflicts or any errors in "system" that could be causing this problem?