@kashelkin mine was the same as yours only stable @ 725Mhz
What 600Mhz firmware did you flash the kernal to cn or en and what date was it,
I done what you said flash the latest 600Mhz version S601 0229cn with your kernel and
all I get is a black screen after the first JXD android screen.... I can recover so there's no probs there...
I use en version but I think it doesn't matter which version is used.
I have the same problem with booting at 1 mhz and I don't know how to solve it for now.
The only thing that helps is to turn console off for 5 minutes. Maybe heat is the reason of these freezes.
@abhoriel, I've tried to raise second and third signatures to 1000 mhz and to leave them as they are and to lower them to 600 mhz but I hadn't seen any difference.
I hoped that it could fix freezes while booting but it didn't.