If you mean those up and down on the core clock graph it might be a signal of instability but it's not a certain proof. Please try to run other test benches and also try to run some demanding games and see if this behaviour is replicated.
Are you experiencing any up and down on the FPS, or stuttering during games as well?
What software are you using in order to overclock the GPU? Seems that your game experience reflects your GPU core clock behaviour. Worth mentioning that the voltage reflects this behaviour as well.
Ok definteley your system seems to be a bit unstable. Here's my stats:
You can see that my voltage is more or less constant and those little spikes are due to my activity while MSI was reading the sensors.
Does MSI allow you to control the voltage of your GPU?