Very strange drive performance

I've been fighting with some hardware raid 5 issues for the last 4
weeks, I'm finally seeing some progress using redhat's dac960 driver
under 2.4 but it is very spotty.

I am getting sequential write bursts of up to 200MB/s, but then for 3 or
so seconds after that, there is zero disk i/o (however the disk is at
100% utilization even when no i/o is being done.. messed up counters?). 
I've attached output from iostat in 1 second intravls.  The drives and
the array are fine, no bad disks, no rebuilding volumes, no errors of
any type... performance is just WAY spotty.

My stripe width is 4 and my stripe size is 128k (I've tried tons and
tons of different configs using different stripe sizes).  The drives are
Ultra320 10k and the raid card is a Mylex extremeRaid 2000 (DAC960
driver).



Thanks,
Ben