Mini Displayport to Hdmi - error in colors!

Hi.

I just bought a new mini-displayport to hdmi cable, for her new Macbook pro 2011 with thunderbolt. After hooking it up to her sony bravia it worked fine (for a while) after 10 minuttes or so, the colors turn into invert or negative.

The cable and tv works fine, with no errors in color, when using my own macbook pro mid-2010.

So is the thunderbolt port on my sisters computer broken?
The recent firmwareupdate for thunderbolt did not cure the problem.

Anyone experiencing similar issues?

thanks in advance for any help!