100% iowait on ubuntu 13.04 vm

I'm running into a problem on one of my ubuntu VM's where io seems to completely stall for about 5 minutes every 10 minutes or so.  During this time, iowait peaks at 100%, then when the stall finishes it goes back down to 0%.  I'm running a cluster with Cassandra, and the other 4 nodes are just fine.  iostat 1 looks like this during the hang:

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
                1.25    0.00    2.26   96.49    0.00    0.00

Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
sda               2.00         0.00        16.00          0         16
sdc               0.00         0.00         0.00          0          0
sdd               0.00         0.00         0.00          0          0
sde               0.00         0.00         0.00          0          0
sdf               0.00         0.00         0.00          0          0
sdb               0.00         0.00         0.00          0          0
dm-0              0.00         0.00         0.00          0          0

any ideas on how to address?

Thanks!

Paul