Tethered using live view restrictions
I did a search a while back and there were a number of people who said they were getting the high temperature warning after videoing for 30 minutes. Now these were people in Florida so the ambient temperature was probably quite high to begin with.
You could try having a fan gently blowing air over the 7D to help dissipate some of the heat?
I wondered why the ball kept getting bigger, then it hit me.