A lot of conversions are done with crappy parts. If you are using a $10 chain, for example, nothing in the world is going to make the bike feel good. Same goes for some of the bargain basement cogs floating around out there. And if you simply reused the old chainring that was on the bike, chances are it isn't meshing with the new chain very well -- unless you're using the old chain too, which would be another clue.
Oh, and frame stiffness really doesn't have anything to do with it.