Is it specific applications? Does the blocking occur while the computer has been on the same network or are you moving between networks? When the blocking occurs if you go into Control Panel > Network and Sharing does your Network Category change (meaning does it say Private or Public network) and does this change when your application is blocked? You could also try installing wireshark and when the application is getting blocked run wireshark and capture the network traffic. Feel free to upload the capture file for review.