[LAD] 2.6.20 scheduler and buffer underruns

We're doing an ARM-based embedded device, which right now is running vanilla 2.6.20. For the sake of simplicity we wrote an OSS driver that's simply double-buffering and writing to the DAC via I2S. We have a buffer underrun problem that is directly proportional to CPU load... no glitches when simply cat-ing a file to /dev/dsp, but lots of glitches when other things are happening on the system. Can anyone suggest tools/techniques/patches for improving the situation? Use an alternative scheduler? Figure out which driver might be turning interrupts off for too long? We're using 20ms buffers which seems pretty generous. I know this is a bit of a noob question, but any advice or pointers to info much appreciated. darren