Hm, if it really is underruns, then it may be fixed by a recent kernel commit (this assumes you're running KMS):
commit dff33cfcefa31c30b72c57f44586754ea9e8f3e2 Author: Jesse Barnes <email address hidden> Date: Tue Jul 14 10:15:56 2009 -0700
drm/i915: FIFO watermark calculation fixes
Hm, if it really is underruns, then it may be fixed by a recent kernel commit (this assumes you're running KMS):
commit dff33cfcefa31c3 0b72c57f4458675 4ea9e8f3e2
Author: Jesse Barnes <email address hidden>
Date: Tue Jul 14 10:15:56 2009 -0700
drm/i915: FIFO watermark calculation fixes