Yes, this issue is just as you describe (non-Cyan Lumia here). There's something with WiFi and BT that Lumia Black started for me, where it will make audio stutter, until you turn off wifi or restart. It's very annoying and happens about once per day. My theory is that some background sync process is hurting performance of the audio, and that process doesn't run when you are not on WiFi. But that's just a guess and foreground WiFi usage seems okay (like when I'm watching YouTube over WiFi). WP 7 had a similar thing sometimes where the phone would slow way down when you started charging the phone, which is what made me think it was some heavy background data thing that is triggered, based on power in WP7's case, or just being on WiFi in WP8 post-Black's case. That's just a guess though.
08-24-2014 01:35 AM