I'll throw some stuff at the dartboard here.
I have a class 4 32gb Sandisk, and I got my Neo about a week after they were released. I have had zero problems here. I don't know if there's a bug with 64gb cards specifically or if its those buying class 10 cards, which is something I can't understand people putting in a smartphone and thinking they can remotely use that. Class 4 or 6 are perfectly capable of capturing 1080p video, let alone lagless switching between media stored on it. Smartphones can't presently utilize the maximum read/write speed of a class 10 card anyway because its not necessary, it makes about as much sense as sticking a USB 3.0 flash drive in a USB 1.* port.
Kmn3 isn't using one like that so I don't know. I listen to music alot, and since that is read from the memory card and there seems to be no way to kill it off completely after exiting the music app, I fire up an app called Kill My Music to fully shutdown the media player and take it off my volume indicator. Wonder if that's what's prevented me from having issues up until now?
If it matters, I did not install the advance release of GDR3.