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Which is precipitated by an occasional then increasingly often pronounced loss of power, right John?
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Yes, I'd say that happened across about 1.5 track sessions of a total of maybe 40 miles.
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BTW: what exactly does advancing the timing do. I seem to have forgotten. Doesn't it add torque?
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Not necessarily. Only if your engine is reaching peak pressures when the pistons are well past the center of the stroke. In that case initiating the combustion sooner moves the peak pressures closer to range of the stroke where piston gets more leverage -- a good thing. But more of a good thing results in the pressures increasing before the crank gets to the high leverage zone when the piston is traveling the fastest. This results in significantly increased pressures and temperatures which can spiral out of control. The uneven combustion also destroys the boundry layer that protects the piston from combustion temperatures which means that the piston temperatures can exceed safe temperatures for the piston material. These hot spots also initiate more pre-ignition. Once that point is reached, the piston starts to burn or melt and I believe can burn through pretty quickly.
I've kept that piston as a reminder of what playing with the ignition timing can do if you don't know what you're doing. To put it differently, my next race engine will have the timing set on a dyno before it goes on the track. That way I'll know exact what is the correct spark timing rather then too much.