Thgis can be a difficult one! as you know but you have looked at one area that seems to me still to be a possibility. A ,number of items and especially the processor have shut down facilities that do not appear on the logs if they get too hot; You note that the temperature under a stress test is 80°C and whilst that is still within boundaries it is a bit hotter than I would like. Are all your fans working, are they all clear of dust (mobo also clear of dust) and are all the vents in the machine clean and are all the heat sinks clean. Depending on what your ambient temperature is it could be that some componenet (memory chip) is closing down because of heat problem. I live in a fairly worm place and many clients have had this type of problem, not easily tyraced but ALL above precautions taken (even moving the computer away from walls to allow better ventilation has cured some of them.
M