Hi everyone, I have the same mid 2012 macbook 13” with both RAM memories mentioned above and with the same problems. I have done each and every possible tips mentioned by all with same results, it works fine and then fails and starts with three beeps. Worthless to say it works beautifully with only one RAM at the top slot. I fixed a big problem that caused it to restart when you picked the laptop with the left hand by inserting one or two credit cards below the RAM slots and the bottom. Finally with both RAMs installed I can use the mac normally for a long time without any problem, but when I let it sleep some times it would turn off by itself and when trying to turn it on again the three beeps will appear or the strips then I keep the start pressed to turn it off and again I try to start it and it would normally. Bottom line, I can live with this problem or buy an 8gb RAM at the top slot to compensate for the speed.