I had the same issue with a Bravia KDL-52VL150. I worked with Sony Support for 10 months resulting in getting a new cable box, they sent me a new Sony Google TV box, a technician replaced the TV mother board (said the HDMI port was bad) and in the end they sent a new TV. The new TV did the same thing so I continued searching the internet and ran across a forum with this suggestion:
Using the Sony TV remote press Menu. In the list of options press Channel then select "Control For HDMI" then select Off.
That fixed it for my TV. It seems that setting is supposed to allow the device the HDMI cable is connected to to be turned on/off with the TV but that does not work correctly resulting in the TV shutting off intermittently and coming back on after 5 to 20 seconds.
Give it a try and good luck.