I doubt the issue is caused by JSTL. In fact, your application server should support JSP 2.1 in order to use JSF 1.2 with JSP views. In fact, Tomcat 5.0 and 5.5 does not support that. What application server do you use?
I am using Tomcat 6.0.18.
Tomcat 6.0.18 should handle this ok. Have you tried to clean temp/work folders?
also if not helps provide additional info about libraries list used by your application.
the only way I got around the problem was to migrate to Facelets as some other users did.
Thankfully my client was already thinking about migrating the application to Facelets and these problems helped on his decision making process.
As what the real problem was, I could not find it.
Thanks for the support,