Diagnosing Hardware Problem - Motherboard or Hard Drive?

Hi! It's my first time posting here and I'm currently confused as to what exact hardware problems have caused my desktop to repeatedly freeze and shutdown.

My specs are as follows:
CPU: Intel i7 920
GPU: Nividia Geforce GTX 660 Ti
Hard Drives: 1. Corsair Force GS 240GB SSD (For OS and Games) 2. A Seagate ST31000528AS HDD (For Storage and misc. data)

My computer has been suffering constant freezes and forced reboots the past two months and I've had no idea whats causing it. The computer mainly freezes when I startup my computer - often freezing immediately after logging in. It should also be noted that the computer freezes and does not display a BSOD. A characteristic of the freezing is that the CPU fans get extra loud and just stays at that volume, however upon checking the CPU it's not actually overheating at all. As such I'm lead to believe it might not be a CPU problem, but more of a harddrive problem - specifically since I run two hard drives at the same time.

The problem is that I've run tests for both CPU and Hard drive and they both report to be working perfectly. I used the Seagate Seatools to scan for errors on both my hard drives and they both passed a full test. I also stress loaded my CPU with the HotCPU program, and my system worked perfectly fine. My computer also runs perfectly during safe mode, which is perplexing.

When I do get my computer to work, I'm able to run high performance games - eg. Heroes of the Storm (Ultra), Starcraft II (Ultra), FFXIV (Ultra), without crashing. In fact my computer seems to crash when I use applications that don't demand alot of resources, e.g. watching video files, surfing the web. As such I dont think the hardware problems lie within my SSD nor my GPU, but rather in my CPU or HDD.

As of now, I've currently reapplied thermal paste to my CPU, which has not helped. I'm also looking to clone my current HDD over to a new one and see if any improvements will occur and if that doesn't work maybe switching a new CPU. However I wanted some outside opinions on what exact problems my computer is facing before I splurge on these new pieces of hardware. Does anyone here have a clue that might help diagnosing what exactly is wrong with my computer?

Much thanks appreciated! :)

PS. I'm not sure if this is relevant, but my sound driver is also a bit wonky. With the audio often cutting in and out for the speakers.
What is the make and model of your power supply and your motherboard?
Have you done any Overclocking?
Are all updates done and drivers updated?
Have you tried to run the computer with just the SSD (your OS hard drive) to see if the problem persists?
I'd definitely look towards the PSU. Try switching it out. I recently had a similar problem - wouldn't reboot, but only when warm - and after testing the voltages and pwr on pin I thought it was graphics but further reading convinces me it's the PSU. Luckily specials abound and July 4th will soon be upon us.