10.7.2, which is in the developers hands right now and should be released soon fixes the issue. In the meantime, go to Energy saver settings, change "Computer Sleep" to Never and reboot. It has totally resolved this issue for me.
When the patch comes out, we can all go back to computer sleep mode if we want! I was plagued with this issue and on Thursday I tried this and I have watched a ton of videos without one single lock up.
A PITA? Yes. When people say that it makes the computer useless, that they are taking it back because it's garbage, it is just riduclous. Here is a easy fix, costs you nothing, and you will be asked to live this way for about a week. Get a grip people. There are far bigger problems in the world.