have you added any new fish lately? ich is a parasite that can exist in the tank and is undetectable to the naked eye. the fish show the symptoms of ich when it is attached and feeding on the host fish. just because you got rid of the symptoms of ich does not mean that you have rid the tank of ich or introduced it into the tank again through new fish. ich is more prevelant throughtout the winter months when tanks have a tendency to run cooler.
depending on the size of your tank, keep a thermometer at the end opposite of the heater. on my larger tanks i have a few to make sure that the temp is constant throughout the tank.
with new fish you should quarantine the fish in a seperate tank. after three weeks of no signs of disease, then add to the community.
when you raise the temp of your tank, you should do it slowly so as not to add stress to your fish. stress is the leading cause of disease in fish as it makes them more vulnerable.
sf