This problem is common and rarely has anything to do with the power supply. Intel XTU does not report this problem correctly. It shows thermal throttling even though the CPU temperature is just fine.
Try running ThrottleStop instead. Open up the Limit Reasons window and you will likely see the real reason your CPU is throttling. BD PROCHOT will be lit up in red.
BD PROCHOT stands for bi-directional processor hot. This allows other sensors on the motherboard to send a throttling signal to the CPU. When one of these sensors goes bad, it will constantly send a throttling signal. On the main screen of ThrottleStop you can clear the BD PROCHOT box so your CPU will stop listening to these fake throttling signals and then it will be able to run at its full rated speed.
If this fixes things, you can either replace the motherboard or just run ThrottleStop. It is a simple solution for this problem.