I recommend you visually inspect the face wiping contacts connecting CPU and memory, especially those on the CPU board plug in that face upwards (possibly marked J700 pins 1 thru 70). It only takes one piece of lint, grit or spec of plastic to keep the gold contact pairs from connecting. A high power magnifier would be good. Any suspicious area could be cleaned with gentle wiping with an ethanol dipped wood toothpick. Conductive metal foreign bodies on either the motherboard or CPU board are another less likely possibility, these usually cause total failure but cannot be ruled out by the observed behavior. So additional inspections and careful vacuuming or air blast cleaning, while minding static discharge risk, might also help. Of course, the SMT connector soldering is also suspect, but I do not know an easy way to visually inspect or even continuity test with an ohmmeter, they are right there in front of you , but they are so *@! tiny and packed in there. [!] !]'>