Is this on the bench or have you tried to fly it properly. Try a quick punch out from a few inches upto about 10ft and see if it can get past the 60%.
The KK is an old FC
It was nearly my first but I got a naze although broke easy they where good at the time. Never heard of this since been on here. I know the KK is obsolete as the devs don't support it anymore along with all other F1 boards.
The only other thing I can think of is spikes from the main line.
Are you using any Capacitors on this build. Can you send a pic of your stack.