My suspicion is a failing memory module which can't be detected by running "Memtest86+". Some modules may only fials when they are cold and works fine when they are warm. Your issue might be similar. There is only one way to detect it - Physical isolation. That is if there is more than one module in the system, unplug all but one and check the system. Repeat the test with all the modules (one at a time). If any one of them is faulty, we can find it.