1. In an implantable medical device that detects a polymorphic tachyarrhythmia of the heart of a patient as a function of measured time intervals between sensed events in a cardiac signal, a method of improving the specificity of discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia comprising the steps of: a) provisionally declaring a polymorphic tachyarrhythmia when at least a first number of the measured time intervals satisfy the polymorphic tachyarrhythmia detection criteria; b) successively sampling, processing, and temporarily storing the cardiac signals to derive a plurality y of data sets of signal amplitudes; c) successively deriving a plurality y of sets of wavelet transform coefficients from the respective plurality y of data sets of signal amplitudes; d) comparing at least selected ones of the wavelet transform coefficients of each of the y sets of wavelet transform coefficients to determine a match count x of the y sets of wavelet transform coefficients that match one another within at least a predetermined match tolerance; e) withholding the final declaration of a polymorphic tachyarrhythmia if the match count x indicates that the corresponding cardiac signals exhibit morphologic stability suggestive of a monomorphic tachyarrhythmia; and f) making the final declaration of a polymorphic tachyarrhythmia if the match count x indicates that the corresponding cardiac signals exhibit morphological instability suggestive of a polymorphic tachyarrhythmia and the polymorphic tachyarrhythmia is provisionally declared in step a).