This is not a complete answer. But perhaps helps.
Almost three years ago we did the following:
ES - ODBC Adapter - StarSQL - DB2 (on mainframe)
This worked pretty well. Stable. Good performance. (Whenever we hit performance issue in the whole integration, it was never this side, database or adapter/bridge).
StarSQL came from a company called StarQuest, who was acquired by Level 8 two years ago.
Given our experiences with ODBC and JDBC, I would tend to try to get ODBC drivers and bridges, and use the ODBC Adapter. In your situation I would try:
ES - ODBC Adapter - ODBC Driver - DB2Connect - DB2
This might help on the performance side, though there are many factors in performance. There are also on stability, though if the JDBC is exactly where the stability issues occur, this might help a little.
Note this is an SI experience reflection. You may want to contact webMethods support to find out the official recommended configuration for connecting to DB2.