First things first - if you have anything overclocked set it back to defaults. Test again.
You likely want to get the following information:
- CPU clock speed over time
- CPU voltage over time
- CPU usage over time
- CPU temperature over time
- GPU clock speed over time
- GPU memory clock speed over time
- GPU voltage over time
- GPU usage over time
- GPU temperature over time
These will tell you if you are throttling and potentially due to what. At the moment it looks likely that the GPU or CPU is downclocking, that seems likely. Its also possible its not that but its the first thing you need to rule out.