This sounds suspiciously like a clustering problem.
 
You do mention that a write is taking place, and glance is showing 50 I/O's
per second.
 
Try to determine which dataset is being written to at the time, and run
HOWMESSY or DBLOADNG against the database in question and look for a large
max.block number associated with a master data set.
 
If this is indeed the problem, use Adager or some other tool to increase the
capacity of the data set in question. From my experience, increasing the
capacity SIGNIFICANTLY offers the best chance of fixing the problem. Some
experimentation may be in order.
 
Hope this helps.
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