Ok! This is now FIXED!!!
Yes, James, you were right on the money: second tear-down today, removed the main-board, removed the cooling fan/heatsink assembly, and yes, the heat transfer pads (one on the cpu, another on what I presume is the gpu) were both "rumpled", like a couple of carpets all kicked up in the middle.
I gently cleaned and polished the shiny faces of the heat transfer surfaces of the processors using terrycloth moistened with rubbing alcohol. I also lightly sanded and cleaned the surfaces of the heat sink where it covers the chips. I applied Antek cpu heat-transfer grease between the heatsink, the cpus and the pads, making sure the pads were smooth and unrumpled as I reassembled it.
I've been pushing the system hard in both Ubuntu and XP, and the narcolepsy has not reoccurred.
It's interesting to me that the Voodoo cpu management driver in OSX does such a superior job of throttling the processor, that it was never pushed to the point of triggering the self-protection suspend.
Anyway, thanks for the encouragement and advice.