1 hour 20 minutes into the flight.  A 40 second blip. All 4 cylinders
showed the same response.  

Guesses:
1. I assume that you are running lean of peak.  You are up high.  You
changed tanks.  There is vapor in the fuel line on the tank side of the
fuel selector valve.  The fuel there is warmer from being in the cabin
and atmospheric (low) pressure in the fuel tanks resulting in some fuel
being vaporized.  As the vapor went through the fuel system, you went
leaner from the vapor and the EGT fell.  I have no guess why it
rebounded above the baseline.  Something caused the engine to meter fuel
slightly richer than baseline.

2.  Fuel contamination, running LOP, will cause a blip down in EGT and
then return to baseline.  I am not sure that it would over shoot.
Again, this probably came after switching fuel tanks with a drop of
water that ran through.

3. If you are running rich of peak, then ask again what caused the fuel
system to meter more fuel to the cylinders and then less.  Would
changing tanks cause a momentary increase in fuel pressure and then a
momentary drop?  I doubt it, just asking the question.  This would be
easy to test, switch tanks several times and then look at the results.

4. If none of these fit, ask if there was a sampling error in the
system?  Not likely, just ask the question and watch in the future.


Disclaimer:  Not an engineer, just a guy who likes to solve mysteries
for greater understanding.

Carl Cadwell
N25CL IVP