Loose or wearing-out probe wire?

No that can't be the explanation. If it were just a CHT or
an EGT reading, sure the probe could be at fault. However since
both CHT and EGT change at exactly the same time I believe one
can only rationally conclude that something happened with the
engine itself. In fact I think the EGT rise is a direct consequence
of the CHT reduction although I can't quite remember how that
connection is made.

~Paul