Your overclock is stable when you do nothing, it is instable when you run a program. Pretty normal.
Overclock a little less till it's stable.
Start with 4.3 all-core and then go up until it's not stable anymore(4.8 GHz max at the moment), clock around 200 MHz back and most of the time you should be okay.
Things you can do to reach a higher overclock:
Delid it and change the TIM between the die and the heat spreader with liquidmetal.
Flatten the outside of the heat spreader (it's often not flat).
Use liquid metal TIM between the heat spreader and the waterblock.
Check Der8auer on youtube, he made a lot of video's how to do this in a save way.