I have a 2015 27" 5K retina iMac which was fine. I installed 8mb of RAM and worked perfectly for 2 months and then suddenly started to crash. The more I use it the worse it gets. I have taken the crucial memory out - it has been rebuilt and had a new logic board yet still does it! Day 1 after rebuild last about 3 hours and then gets progressively shorter until it it will barely turn on. If I leave for 12 hours get better again. I need to unplug it and leave for 15-20 minutes before it will restart. So I don't think it can be the memory otherwise surely it should go away. Seems more like a mac fault.
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