Try running just a dual-channel memory configuration or single stick in each slot. It may be a little laborious but you may identify either a bad stick or a bad motherboard slot.
You've already re-seated the CPU but you could try that again - inspect the pins on the board and pads on the CPU carefully and see if there is any damage/fluff that may be causing a bad connection.
Does the little OLED screen show a 2-letter code next to the "Memory" message? Search the net/look in the motherboard manuals for "QCODE xx" (replace xx with whatever your 2 characters are) and you may find out more about the exact nature of the Memory problem.
You could also try booting the system and while its starting up, press the little red MemOK! button, near where the front-panel connections go. That should force the board to try running through a known list of safe/known working memory settings, one of which should work. May take a couple of tries to get the right timing of when to press it (I've never used it before but read about it, worth a try).
Good luck mate.