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#21
Check your RAM sticks are all plugged in correctly. Take them out,clean the slts with compressed air (any cleaner of your choice) and plug in the RAM sticks back one at a time. Wiggle the sticks a bit, if they wobble alot, then you'll have to get a replacement.
Put in one stick, boot up computer to check computer has recognized that stick, then put in the second one, boot up computer and check if its been recognized and so on untill all sticks are securely in.
Try and switch the slots of the RAM sticks
Also, Check the BUS speeds of the ram sticks. how many MHz are they running at?
Speccy - Download
download that free program and click the RAM tab, Next to max bandwidth it will tell you how in () how many MHz there are