1. A method of discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia of a heart of a patient having a cardiac signal, comprising the steps of: first deriving a plurality of data sets of signal amplitudes of at least a first number of measured time intervals of the cardiac signal satisfying a polymorphic tachyarrhythmia detection criteria; then deriving a corresponding plurality of sets of wavelet transform coefficients from the plurality of data sets of signal amplitudes; comparing at least selected ones of the wavelet transform coefficients of each of the plurality of sets of wavelet transform coefficients to determine a match count of the plurality of sets of wavelet transform coefficients that match at least one other of the plurality of sets of wavelet transform coefficients within a predetermined match tolerance; then declaring a monomorphic tachyarrhythmia if the match count is greater than a morphologic stability threshold; and declaring a polymorphic tachyarrhythmia if the match count is less than a morphologic stability threshold.