Q: iMac with Yosemite 10.10.2 installed, won't boot at all.

Good day.

I own an iMac 27" (late 2011), and recently I've upgraded from Mavericks to Yosemite. It ran perfectly for the past 2 months.

A week ago, I was watching a video clip on QuickTime, when my iMac suddenly froze, and the screen went completey white and unresponsive. I waited a couple of minutes to make sure that nothing is happening, and then I turned my computer off then on. The problem is that my iMac refused to reach the login window. It showed the progress bar, and when it reaches about 1/3 of the loading phase, the screen goes completely white, then a few seconds later it reboots by itself and the same problem occurs again in an infinite loop.

I tried to boot into recovery mode (cmd+r), and also (option+r)  but it never worked, it continues booting normally then gets stuck again.

I tried to hold shift when booting to enter safe mode, but again nothing happens and it continues to boot normally and then it gets stuck.

I tried to hold D when booting, but nothing happens. I tried the (cmd+s) and tried to fix the volume, but that didn't fix my problem.

I tried holding Alt when booting, it gave me the option to enter the recovery disk 10.10.2 with the option to choose a wifi network. When I click on the recovery disk icon, it continues to boot normally as before and eventually gets stuck again.

I tried to run the internet recovery mode, and when the whole progress bar was done, the screen went completely white again and I couldn't reach the disk utility window that should have appeared once the internet recovery mode is fully downloaded.

Then suddenly, when I tried to restart my computer, it finally reached the login window and the iMac booted successfully.

It kept running smoothly for 3 days, and again yesterday night, while watching another video clip on QuickTime, the same thing happened, and now I'm stuck again with the same problem.

I tried all of the steps again that I did before, but the problem still persist.

What seems to be the problem? and why can't I access the recovery disk or safe mode? and why doesn't the internet recovery mode takes me to the disk utility window once it finishes preloading?

Thank you in advance for your help, as I urgently need to fix my iMac.