Computer Crashing - No BSOD - Lots of Information!

Hi guys.

First I'd like to say this is probably going to be a long post with a lot of info, some of which I may forget to include. Also, I'm not exactly the biggest computer genius out there, so please have patience!

I built this PC around 3 months ago and it worked fine for around 1 week. After that, I would occasionally get this problem where the computer freezes (the screen completely stops) and can only be fixed by the power/reset buttons:

• The screen stops completely, but no visual defects are caused.
• The mouse stops moving. Clicking the mouse buttons does nothing.
• The 'num lock' and 'caps lock' keys on the keyboard become unresponsive.
• Sounds becomes very distorted, but if you listen closely you can still hear the sound being played very slowly.
• No noticeable difference in the speed of the fans in the case before or after crashing.
• Strangely, the animation the mouse makes when loading something (the blue circle) still works
• No dump files are made.
• The system does not start responding if I wait.

A lot has happened since then in terms of troubleshooting and discovery so I'll list everything in chronological order:

• I ran multiple MemTests (MemTest: RAM reliability tester for Windows) overnight multiple times with no errors found.
• Used my computer normally with 1 RAM stick at a time, then tried using both sticks but in different slots.
• I did stress tests on my CPU and GPU with no errors or crashes caused.
• I updated my mobo and GPU drivers.
• Reseated my GPU.
• I changed some settings on the BIOS, such as changing the RAM speed between 1333 and 1600MHz and running it on performance mode.

None of these made any difference.

A few days later I got a bitcoin mining virus which was causing display errors. Eventually I got rid of it, but I noticed that the crashing actually stopped during that time. It also stopped happening for around 2 weeks after I got rid of the virus.

Stupidly, I decided to upgrade my GPU drivers again and the crashing started again. It wasn't noticeably different from the last time. This time I took these measures:

• I tried to 'roll back' the upgrade using the driver roll back feature.
• I tried to uninstall the driver completely, clear the remaining files using a driver cleaner and reinstall the same driver on safe mode.
• Again, I uninstalled the driver, used a driver cleaner and installed an older driver.

Eventually I got tired of making no progress and gave up trying to fix it.

======================================

A few days ago, I bought an SSD (Samsung 840 EVO 120gb) and installed my OS onto it. I then connected my 2 HDDs and formatted them both. Unfortunately I didn't think to do any crash testing at the time, as I had assumed the issue was a driver issue and that formatting the HDDs would fix it.

After installing all the drivers and updates, I started using the computer as normal to find that the problem remained. However, this time it is MUCH worse, with the crashing happening upwards of 20 times a day. Fortunately though, this has allowed me to identify some things:

• The crashing happens a lot more when I multitask.
• The faster I start multitasking, the quicker and more likely the computer is to crash. I am actually able to induce crashing by starting a download and opening WMP together. Opening them 30 seconds apart is much less likely to cause a crash, though.
• Multitasking while downloading files seems to make the problem even worse.
• The crashing rarely happens when idle.

Here are some other things I've noticed. Unfortunately, I know I've forgotten some things but I'll post them when I remember:

• The extent to which a program uses the CPU, GPU and RAM doesn't seem to matter. A game does not cause crashing more frequently than web browsing, for example.
• The inside of my case does not seem to be very dusty. I clean it around once a month with a brush and compressed air.
• I've never had any kind of 'accident' with a part which could have had the potential to cause damage.
• Sometimes, pressing the restart button doesn't actually work. Instead, the system turns itself off suddenly about 10 seconds after crashing.
• I ran MemTest86+ on fail safe mode (1 pass, 0 fails). If I force all cores, it crashes at around 23% in every time. All numbers and symbols stop changing and all keyboard commands stop responding. However, the flashing + in the top left corner does not stop.

That's all I can remember for now. I'll be sure to add any other information as an edit to this post. Videos coming soon.

Any thoughts?