Blocking on DBCP connection pool (open and close connection). Is database connection pooling in OpenEJB pluggable?

We use OpenEJB on Tomcat (used to run on JBoss, Weblogic, etc.). While running load tests we experience significant performance problems with handling JMS messages (queues). Problem was localized to blocking on database connection pool getting or releasing connection to the pool. Blocking prevented concurrent MDB instances (threads) from running hence performance suffered 10-fold and worse. The same code used to run on application servers (with their respective connection pool implementations) with no blocking at all.

Example of thread blocked:

Name: JMS Resource Adapter-worker-23
State: BLOCKED on org.apache.commons.pool.impl.GenericObjectPool@1ea6b4a owned by: JMS Resource Adapter-worker-19
Total blocked: 18,426  Total waited: 0

Stack trace: 
org.apache.commons.pool.impl.GenericObjectPool.returnObject(GenericObjectPool.java:916)
org.apache.commons.dbcp.PoolableConnection.close(PoolableConnection.java:91)
   - locked org.apache.commons.dbcp.PoolableConnection@1bcba8
org.apache.commons.dbcp.managed.ManagedConnection.close(ManagedConnection.java:147)
com.xxxxx.persistence.DbHelper.closeConnection(DbHelper.java:290)
....

Couple of questions.

1. I am almost certain that some transactional attributes and properties contribute to this blocking, but MDBs are defined as non-transactional (we use both annotations and ejb-jar.xml). Some EJBs do use container-managed transactions though (and we can observe blocking there as well). Are there any DBCP configurations that may fix blocking?
2. Is DBCP connection pool implementation replaceable in OpenEJB? How easy (difficult) to replace it with another library?

Just in case this is how we define data source in OpenEJB (openejb.xml):

<Resource id="MyDataSource" type="DataSource">
  JdbcDriver oracle.jdbc.driver.OracleDriver
  JdbcUrl ${oracle.jdbc}
  UserName ${oracle.user}
  Password ${oracle.password}
  JtaManaged true
  InitialSize 5
  MaxActive 30
  ValidationQuery SELECT 1 FROM DUAL
  TestOnBorrow true
</Resource>

Resolved issue with dbcp blocking by changing pool configuration (openejb.xml):

TestOnBorrow false

Thank you, Andy, from OpenEJB team!