Hi,
I think your websphere is causing some operations in db2, try reading the db2 websphere tuning guide from the
www.redbooks.ibm.com that will immensely help you.
Also try setting the connection and orphan connection timeouts in websphere.
You will need to closely monitor what's happening at the db2 as well as websphere end to exactly narrow the scope of the issue.
HTH.
Nitin