Running an imac 7,1 aluminum 2GHz core duo. Originally had 1GB ram. Added a PC2 5300 DDR2 667MHz 1GB Kingston RAM module to the free slot so I could run El Capitan. The machine was working OK at first, not snappy quick but running, and after a short while started running very slow, choppy, freezing up, basically was non-functional. So, I thought I'd try more RAM. The machine has 2 slots. I purchased 2x2GB RAM sticks of the above configuration. If I place both new chips, the machine will not boot, no chimes, I hear the first mechanical sounds of a boot but then nothing. If I replace the original 1GB ram stick in one slot and one new 2GB stick in the other, it works fine. Actually running faster than before, showing 3GB of RAM functioning. I would very much like to have all 4GB of RAM but without the factory installed chip in place I can't get it to boot up. After extensive searching I have found others with this problem but no good solution offered. The chips are 100% seated correctly, both slots are working. I have tried each of the 2GB sticks and they both work when paired with the factory stick, but not when paired with eachother. Any thoughts?