I did a benchmark on my cpu and i wonder: Is it normal for the cpu utilization to go from 100% to around 70% in 1 second and then go back to 100 again?
No it's not bad for the PC. It is common for the CPU utilization to jump up and down when under load. The only thing that might be bad, is if you have poor cooling and your CPU is constantly very hot (80< degrees C) then it may shorten the lifespan of your CPU.