I don't understand why all these people are saying to return it when their have been about 3 or 4 different people in this thread say they have experienced the exact same thing.
If thats the only issue you have I wouldn't worry about it. It sounds like it has something to do with your phone being very low on battery when you do it, was yours?
I mean sending it in for a warranty replacement your not guaranteed a new phone it could have problems as well, if the one he has is perfect other then it does that screen once in a while I would be happy with that.
Now if your in your 30 day window of buying the x then I would definitely take it to the store and see what they say.