Some of the reasons which I see it as follows.
01>Right version of type4 driver
02>Too many thinkings in static methods or too many calls in init() methods
03>Did you tried trimming of Jboss
04>What JDK you are using
05>what is the max and min connection setting in ora-ds.xml
06>Which version of Hibernate you are using
07>Can you try to start the Jboss without polling to DB, see what is the difference
08>What is the per gen setup you are using
09>Are you using default class loader or you have changed something there
10>Please disable hot deployment if its on
Try these options I am sure you will able to pinpoint the problem.
Thanks,
Ruchir Choudhry