99% of activity is normal, the drop to 70% however is not, it could be that your GPU is overheating and it throttles itself down until temp reaches an acceptable level. It could also be that your CPU is choking your GPU in a very intensive situation for the CPU. Or your PSU is being unable to deliver enough power to the GPU in very intensive situations...
Lots of possibilities as you can see, we need more info to be able to find which one is the problem. Start by posting your full pc specs, on which games at which video settings you're having the issue, and if possible logs of your CPU/GPU temps, loads and power usage (HWMonitor Pro is great for this kind of info).