I've been having a problem with my dsm520 dropping audio every few minutes for what I would estimate 1/10 second. It only seems to happen using the digital outputs (either coax or optical) and only happens when reading an mpeg 4 file. Specifically, the mpeg4 is an XVID.
I recently found that using virtual dub to re-write the file with 500ms of audio preloaded (option is under interleaving) seems to fix this problem. The DSM510 was in fact able to play these files correctly without being "changed", but I hope this helps others who may have also experienced this.