The defaults for max connections are incredibly low. This is standard tomcat connection pooling stuff, I would definitely try increasing the default, see if that helps. you can monitor concurrent connections on the db side - just because you have 100 connections to the db it doesn't necessarily mean they'll be all used at once.
Also; Are you using mysql? You should try the c3po pooling driver it handles timeouts and things better than the standard driver so you shouldnt ever get dead connections sitting in the pool.