I agree on the water change front if it makes the PH, temp, hardness swing a lot OR if there is Chlorine or Chloramine present in the water.
ALSO
Other than big water changes are you testing the water? If the filters are not up to the job or the bacteria is being killed by Chlorine or Chloramine in the water change you may be getting Ammonia spikes in between.
How big is the tank, how old is the water i.e. is it recently cycled and how are you filtering it?
Was the fish shipped recently without quarantine and how long was it at the shop before you bought?