Classic server it might help on robustness : each connection will be isolated (where a crashing server doesn't take down the other connections and it will use more resources ) , but for scaling it uses the same  architecture as SuperClassic (no shared cache) so cpus are used the same way 

http://www.sinatica.com/blog/en/index.php/articles/firebird-superserver-classicserver-or-superclassic

http://mapopa.blogspot.ro/2010/05/what-to-choose-from-fourth-firebird.html

also we need more details , io/memory/cpu stats , also firebird stats and logs with actual errors