Overheating? i7 4770 CPU running at only 3.64GHz?

I just bought a new HP desktop with i7 4770 CPU, running some CPU intensive applications. I see in the task manager, the CPU usage is at 80-85% usage at 3.64GHz only. This CPU is supposed to be running at 3.9GHz turbo, is my CPU defective?

The PC randomly shuts down nearly every hour. Only message I got is "your PC ran into a problem" but there's no other message. It did list 3 files that should be sent to Microsoft for diagnosis but I declined to send them.

The i7 4770 is actually rated @3.4GHZ and with the turbo boost it can go up to 3.9GHZ. Depending on what application you are running and the number of active cores it uses, it can range from 3.7GHZ - 3.9GHZ.
You can look at the table from this link for more explanation: Intel® Core™ i7 Desktop Processor — Intel® Turbo Boost Technology frequency table

If the PC randomly shuts down, it may be due to drivers software that cause the system to crash, faulty memory etc...

Spontaneous shutdowns could also be caused by overheating or defective or inadequate power supply. CPU, not likely.