
Did you check that it is not a faulty cable? This happened to me.
The cable in question gave me intermittent sound outs and video outs too. Not necessarily both at the same time - but that too happened on a few occasions. Your problem sounds very similar.
I eventually traced the fault to the HDMI cable that came with my PS3. I bought another, made by Philips incidentally, and the problem has never come back.