this is no jboss-problem and dbheap was not the right parameter. (some default ddparms are to small)
play with following parms:
stmtheap = 8192
applheapsz = 1024
appl_ctl_heap_sz = 1024
will also be nessesary. look to the db2-errormsg.
hope that helps
You may need to set some jboss read-ahead parameters so it loads for fewer rows at once. Look in the persistence forum, others have had similar problems.