I have the same problem like hal rockwell, so I will repeat his question here:
I have a Sandisk Sansa Fuze+ 4GB with 64GB SD card running the latest Rockbox revision. After about a half an hour of playback, or at some Songs everytime, the player starts to lag - it doesn't respond to the buttons and the music stops for a moment and than resumes again. It repeats itself every 15 seconds or so. I happens only when I play MP3 files and doesn't happen with any other format I tried. With earlier Rockbox revision everything was fine.
Please can you help me? This is making me crazy!