I hope they fixed this for you, I've been having similar problems, but my screen has been flickering yellow.
I've sent it to them under warranty twice already and its with them for a third time now...
They saw it first time and just flashed some new software and then sent it back still faulty, the second time they sent it back saying they couldn't find a fault (to be fair it doesnt happen all the time and theres no way of forcing it to happen that I can find).
I've made a number of angry phone calls and its with them again. I loved this phone until this started happening. My next phone wont be a HTC.