I have the same problem.
It happens in 16's and 32's GB microsd but not in a micro sd of 1Gb. Also what I can see, is that happens when I chose to play a song of the dynamic play list while I am listening to another song. To avoid that the Sansa freezes, I have to pause the song that I was listening and then play the other song, but this doesn’t work all the time.
I tried to uninstall and install using rockbox utility but that didn’t help, also I tried installing the firmware manually and the bootloader with rockbox utility but didn't solved it (I don’t understand how to install the bootloader manual in the manual so where I can find information for dummies?)
Please, if someone can give me a solution, I will be pleased to try any idea.