oddball RFI problem

You have an RF leak from a com coaxial cable connector
somewhere. Probably a bad ground at the connector. This is a common
problem with electronic voltage regulators, as they are sensitive to
much smaller currents than mechanical regulators and will misread
interference as an overvoltage condition, and will drop the alternator
offline when the mike is keyed if there's any RF leakage.
Look at the connector at the antenna. Condensation in the
aircraft ceiling will corrode it.

Dan