Hi Ben,

I suggest you to compare amount of queries for each node. May be the problem is on the client side.
Yoy can do that using JMX:
"org.apache.cassandra.db:type=ColumnFamilies,keyspace=<YOUR KEYSPACE>,columnfamily=<YOUR CF>","ReadCount"
"org.apache.cassandra.db:type=ColumnFamilies,keyspace=<YOUR KEYSPACE>,columnfamily=<YOUR CF>","WriteCount"

Also I suggest to check output of "nodetool compactionstats".

--
Alexey