New RAM not working on iMac 2010

Hello,

I own a 21.5' mid 2010 i3 iMac and, after all this time, it was time to upgrade the RAM memory.

I've purchased 2x4GB Patriot SODIMM PC3-10600 1333MHz, 204 pin, as instructed by the Apple Memory Upgrade webpage (Install memory in an iMac - Apple Support).: 4 GB RAM SO-DIMMs of 1333 MHz DDR3 SDRAM.I've installed it according to instructions plus the factory 2x2gb Samsung RAM sticks. After a restart, everything worked beautifully for about 6 minutes. The computer identified the new sticks and showed me a full 12GB available. Which was great, since Yosemite + a couple of browser windows and Spotify alone takes up to 3GB. I have enough to spare. However, it restarted itself, and again, and again, and again.

I decided to test only the new RAM and booted it with the 8GB Patriot combination. It froze at restart and only fully booted once, but it restarted less than 2 minutes later.

Result: now I'm back to the original 2x2gb combination. Still facing the lack of memory and without understanding what's wrong with the new RAM I've purchased at Fry's. Could it be a matter of compatibility? Does it have to be another specification? Of course I know it may be a matter of malfunction of the hardware, but I'd like to sort this out before making another run to the store and risking another model or brand.

For reference: I've tried it before, but, due to lack of information of the salesperson, I was sold a 1x8GB RAM stick, which is not supported by my model. So I replaced it by the 2 in question.

Thanks for any help.