Somewhat related, I've found the proximity sensor to be extremely buggy without headphones (holding phone to head). Constantly knock the mute button and if I try to use the phone during a call (while on speaker), the screen turns off (proximity sens...
Bought mine from the ASUS US site and having the same issue. Thus far, only while idle, happening 1-2 times a day, though not everyday that I've noticed. Just updated to the latest software, hoping that helps.