Mitsubishi 1080P HD TV tripping off.

I have a mitsubishi 1080P HD TV. I use an HD cable card from my local cable company. The problem is that the TV will trip off when I'm on the HD channels. I happens at any time (sometimes it takes hours) but will trip off. I've had the local cable company change the card several times to no avail. After the TV stops blinking and resets, I can go back to the channels again, Sometimes it will trip over to a non-HD channel (2). Why does this happen? Ive been told its everything from a strong cable signal to a weak one. My signal comes in right at -1 to +1 so that can't be it. Could it be something internal that heats up and breaks a connection? Need some suggestions. Dan