This is a known bug on certain ipods (4G greyscale and Color, and 1st Generation mini), which causes random crashes when Rockbox attempts to change the speed of the CPU in your ipod (CPU frequency scaling).
The workarounds for the moment are to either disable CPU frequency scaling (which means modifying a configuration file and building your own version of Rockbox from source, or downloading such a build from someone else), or to apply the "COP" (co-processor) patch to the source and compile a build with that patch (it seems to help with the freezing).
Builds with both the COP patch applied, and CPU frequency scaling are available for download in the "Unsupported Builds" forum.