Nobody has mentioned the possibility of a momentary mag problem.  I'd expect
all EGTs to rise if one were to do a mag check at cruise power.  With only
one plug firing in each cylinder the flame front takes longer to propagate
across the cylinder.  This has the effect of retarding the timing and
causing the exhaust temps to rise.

On the other hand I'd also expect a change in the sound of the engine during
a period of one mag operation.

However, one mag operation is a possibility, so maybe you should check the
mag wiring to make sure the leads are tight at the mags and at the key
switch.  Anything else goes beyond my knowledge of mags.  Maybe there's
something else to check or watch here.

Hal Woodruff