ipod classic rockbox is not an official release yet. its barely stable enough to use. playback needs work, battery life needs work, and they havent even figured out how to make it dual boot yet. Give it some time, wait for a stable release. otherwise, dont complain, just contribute to bug reports and development if you want to use it that bad on your classic.
on the ipod video (5g/5.5g) it works just fine. intermittent pauses on that model can probably be diagnosed and fixed. i believe theres a setting in playback on how much of a file you want to cache while playing. That number needs to be large if you are playing flac files. caching 2mb of an mp3 file will pretty much ensure that abotu half the song is ready for playback, but caching 2mb of a flac file will barely ensure that 1/4 of it is ready for playback, savvy?