The game also runs choppy for me, even though my processor is barely being used. I think you should add an icon in the game that shows whether we are playing perfectly or not. I have finished a few songs "perfectly" but only gotten a gold star from them. Maybe you should make the buttons that you have to hold a little more forgiving if that is the problem.
There is no doubt that hard mode is harder now. I only tried it briefly but on level 4 the timer starts about 3 seconds before you can do anything, making it harder than intended.
Do you have a "game complete" setting for the API? I'm curious about how many people are finishing the game. I think you are going to have to make normal mode easier if you want the rating to improve.
Maybe on normal mode you could start with one or two more lives than you currently get. That would make things easier for people that are quitting due to the difficulty. I'm not requesting that myself, but if you think people are getting frustrated then it might not be a bad idea.
Urchin's comments