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Recently I purchased 2 RAM carriers from a local supplier:

Micron Memory Upgrade 8GB 1867MHZ DDR3 SO-DIMM PC3L-14900S (8GBx2)

I wanted to upgrade from my current 8GB to 24GB.

However, a few days after installing, I ran into the screen going weird (for a few seconds). Please see the attached picture.

In addition, today the screen went blank for a few seconds, then came back but I couldn't interact with it at all, then I had to manually restart it by pressing the on button. And tonight, my iMac just restarted itself for unknown reason.

I used Memtest86 to scan for errors (for the whole system). After 6 hours, it found nothing, RAM included. I also used Rember software to double check if RAM are working probably, and again, found no errors. Lastly, I used Mac's diagnostics by pressing D when the computer is turning on, and it found no errors.

These issues never occurred during the 6 years I've been using it, only almost right after I installed these 2 pieces of RAM, so I assume the problem is the RAM. What could possibly be the problem? Is it the RAM? The RAM slots? Any advice is appreciated.

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You could always take out the RAM and see if the problem still occurs. It could just be a coincidence that the problem started after the RAM was installed, in which case you've got another problem!

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Check that the two 4GB RAM SO-DIMMS specs match precisely to the two 8GB SO-DIMMS. Specs should be exactly the same except for the RAM capacity. The RAM you bought is definitely within spec.

Micron is very good RAM. Apple has used Micron RAM in the past. It's still possible you have some bad RAM and you would have to run the tests for many hours perhaps days to catch a failure when it happens. Definitely try taking out the two 8GB memory SO-DIMMS and see if the problem goes away after a few days of heavy use. Also try just the two 8GB SO-DIMMS by themselves. Then try an 8GB and a 4GB SO-DIMM and swap for the other pair. To trouble-shoot which RAM stick is bad.

Try putting the RAM SO-DIMMS in alternating slots. Many computers use alternate slots as channels. Slot 1 & 3 and Slot 2 & 4. It might make a difference when using different sized RAM SO-DIMM's. (Apple doesn't document this, it's just something many computers do with the slot channel arrangement) Apple might not actually do this, it's hard to tell. They list bank0/DIMM0, bank0/DIMM1, bank1/DIMM0, bank1/DIMM1 in the System Information -> Memory view. But it is worth a try to alternate the sizes.

4GB 8GB 4GB 8GB

Instead of

4GB 4GB 8GB 8GB

It will still run best with all four slots with the same sized SO-DIMMS.

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