I ended up installing AlienBBC because there are some files in it that are needed on the Mac for transcoding WAV files. I wanted to transcode WAV files because I wanted to use SlimRoku to tune XM Radio streams. I was having a similar problem to you. There are a couple of things to try:
1.) Transcode the RTSP and WAV streams in AlienBBC to FLAC instead of MP3. I had all sorts of problems with getting the MP3 transcode to play correctly on SlimRoku. It would work fine on my Soundbridges but not on the HD1000. I installed Soaif's FLAC codec from the Sonica website.
2.) Increase the amount of data buffered in SlimServer.
3.) Take 2 asprin and call me in the morning!