I'd say valve springs would be a pretty good guess.

When my Magnum had valve spring issues related to boost - it would rev to 7K at 6 psi of boost but wouldn't rev past 5500 at 12 psi of boost. I kept thinking it was ignition until after I had replaced every single ignition component in the chain. I just knew it couldn't have been my valve springs because they were new... until I actually checked them and saw that some were junk. New springs and it'll rev past 7K at 18 psi of boost - no problem.