...to ascertain the source of these mythical interrupts...

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I'm having trouble imagining memory misbehavior that would not eventually cause a bomb. But I am beginning to wonder about the video card, network card, keyboard and mouse that were presumably common to the tests. Does the mouse point/track smoothly, and does running keycaps from the apple menu show any stuck repeating keys? If there is any choice of video resolution, does changing it change the time loss rate? Booting without the network card, does the problem remain? "Borrowing" another keyboard, mouse, video card and some other memory, and/or trying less memory are other possibilities.