Flashing/Freezing screen - iMac (24-inch, Early 2009)

Hello:

Tech specs: iMac 24-inch (Early 2009), 3GHz, 8GB Memory; running OS X 10.11 (El Capitan).

I'm experiencing strange flashing/freezing screen issues in last months… My computer is considered an absolute machine but I'd like to find the source of the problem and consequently a possible solution.

The Flashing screen occurs "randomly"; it begins with a quick flash then it goes fully glitchy until I restart the computer pressing button.

The Freezing screen might occurs also "randomly", mainly while watching a video; the hole computer freezes (even the cursor) and the video audio gets in a loop.

I use Adobe programs every single day. I thought the origin could be a malfunction on the GPU but I did an AHT (Standard and Extensive) and no problems were found…

That's why I'm using quotation marks on "randomly", because I think that most of the times the glitch occurred, I was playing a video on YouTube…

So, can it be a problem with Youtube and/or Firefox?

Thanks in advance!

h