ELETTRA is generally working in a multi-bunch filling mode which gives rise to an instability if a parasitic higher order cavity mode (HOM) is coupled to a multi-bunch oscillation mode. Longitudinal instabilities, even if not destructive, are particular harmful for synchrotron light sources, since they increase the energy variation of the beam and deteriorate in this way the quality of the light spectrum multi-bunch instabilities in ELETTRA are cured by tuning the temperatures of the four independent cavities. The effectiveness of the method has been verified by monitoring the spectrum of the SuperESCA beamline. The relation between HOMs and cavity temperature and the variation of this dependence with cavity and machine parameters has been studied. Operational experience and dedicated measurements are presented and compared with theoretical predictions.