I have searched this issue and have found several threads some 50+ pages long but I could not find a soluition. I currently have a macbook air 13" mid 2012 model.
Problem: I was using a thunderbolt to HDMI cable to watch movies on my SONY Bravia tv and it was working fine one day and after I updated my software to mac OS X 10.7.5 it stopped working. All i get on my TV is "no signal" and the screen on my MBA starts to flash blue every now and then.
I took it into the genius bar and they plugged it in to their system and it seemed to work, so the guy said to get a new cord.
I ordered a new cord and tried it at home and its still not working. After searching the forums I see that alot of others seem to be having the same issues and I am pretty sure its related to a software issue seen in the thread below.