It started with this symptom for me and then phone started rebooting everytime I open camera app. Factory reset did not help either. Turned out to be a motherboard issue. Got it replaced at service center under warranty.
I too can confirm that this is an issue with 6z from the day I've purchased the phone. FM radio works fine for the first few mins (5 to 8 mins) and then suddenly the sounds becomes distorted/muffled until you restart the radio altogether. Same proble...
