Try using ThrottleStop.
https://www.techpowerup.com/download/techpowerup-throttlestop/
Is the Limits button unlocked on the main screen? I believe your CPU supports this feature. The Limit Reasons data will explain why your CPU is throttling. If it shows BD PROCHOT lighting up red, try clearing the BD PROCHOT box on the main screen of ThrottleStop.
Post some ThrottleStop screenshots of the main window, the FIVR and TPL windows while your CPU is throttling. There should be a clue in that info.
Make sure SpeedStep is enabled in the BIOS. People that recommend disabling EIST or SpeedStep do not realize that this can cause a CPU to get stuck at a low frequency.