The linear stability of high-toroidal-number drift-ballooning modes in tokamaks is investigated with a model that includes resistive and viscous dissipation, and assumes the mode frequency to be comparable to both the sound and diamagnetic frequencies. The coupled effect of ion drift waves and electron drift-acoustic waves is shown to be important, resulting in destabilization over an intermediate range of toroidal mode numbers. The plasma parameters where the assumed orderings hold would be applicable to the edge conditions in present day tokamaks, so these instabilities might be related to the observed quasicoherent edge-localized fluctuations.
