First off, there is no "stable" version of Rockbox for the iPods. It hasn't been released yet.
Secondly, 4th Generation iPods have a known crashing problem caused by CPU scaling, which is most likely whats causing the problems you have with crashing. Try compiling a build with cpu scaling disabled to avoid that, or find a precompiled build in the unsupported builds forum.
As for the sound in the left channel, could this be a failure in the modified hardware? Nobody else has reported this.