Hey thanks for the reply MrSweet...
I have re-done the paste on both GPU and CPU twice now. Upon dis-assembly I inspected my work and it was good - full spread of the paste to the block with a slight amount of extrusion on the sides. All four block screws tightened in proper sequence and torqued nicely. I was very careful to reapply the gel-pads that keep the boards from contacting each other in the right places.
But, actually I have removed the GPU card completely and running on the on-board video. It doesn't make a difference. From my observations I would say that either the thermals are wrong or the shut-off table is messed up. Strangely HWiNFO64 thermal readings show ~30oc for CPU thermals. Only the PCH reading shows 47oC at startup but that has never changed. I have read a bit on this measure and I think it is the bottom end of the scale for this thermal sensor. None of the readings ever warranted an overheat event. Even the fans do not turn on at this point, although I have manually set them both to full using the HWiNFO control and they work great.
Still shuts down though - Fans or no fans.