I finally found some time to take a closer look at my EGT anomaly.  I also inserted the chart as a graphic in this message so the Excel-challenged could participate in solving the mystery. 

Several of you suggested the blip was caused by some electrical issue, so I overlaid the buss voltage trace on a scale that put it in proportion to the EGT traces.  You can see that there was indeed a .4 volt "spike" that coincides with the 30-35 degree drop in EGTs.  But there are other voltage "spikes" that don't seem to have affected the EGTs at all.  Also, what may not have been apparent before is that the EGT excursions lasted for about 48 seconds (there are six seconds between each data point, that is the sampling frequency I have my EDM-700 set to).

I say again that I am sure I wasn't doing anything to the engine or fuel controls at this point.  I don't remember noticing any surges or stumbles in the engine.  The gear pump may have chirped at this point, but why would that cause a 48 second excursion in EGTs?  Any new ideas about what happened here?  Also, now that I see the voltage trace on a larger scale, should I be concerned about how ragged it is?  I have a generic Japanese automotive alternator (35 amp, I think) controlled by a generic automotive-type regulator with a Zeftronics overvoltage relay.  The battery is a 2-year-old Odyssey 925 with about 100 hrs. on it.  The only variable load on the buss at this time would have been the strobes, which cycle about every 2 seconds.

Lee "Moondog" Metcalfe
N320WH - LNC2 - IO-320+ - 522 hrs. - Kansas City