Our gas heater does not seem to stay on. We have the temperature set with a timer and to a certain temperature. The heater starts in the morning at the given time, but turns of a couple seconds later, than starts again, turns of again, and this goes on and on. Which part of the heater need to be replaced? My husband changes the filter, but that does not seem to make a difference. Any idea?

It could be any number of things but first have your thermostat checked. It could be that there is nothing wrong with the furnace itself. The furnace will try to do what the thermostat is telling it to do.