My tomcat 5 server running on centos frequently (several times / day) produces the following error:
Apr 7, 2011 11:02:30 PM org.apache.catalina.loader.WebappClas
Newer versions of Tomcat require that you put JDBC driver JARs in the Tomcat /lib directory, not your WEB-INF. And there should only be one version in that directory - the version you want to use - and no others.
Since you're using Tomcat 5, I'd recommend putting the JAR in your server/lib directory.
I don't know if this is the root cause of your problem, but it's worth a try.