We have a heating/AC unit that was installed about 20 years ago. The furnace unit is in the attic. We experience kind of an odd problem in the winter when the outside temperature is cold (below the mid-40's F). Our furnace comes on in the morning, heats up the house, then shuts off where the thermostat is set, but if it tries to come on again up to about an hour and a half after it shuts off, it won't stay on. The furnace fires up, then when the blower comes on, the furnace shuts off. Again, this only happens when it is cold outside. We have adjusted the thermostat, but it doesn't seem to help. We even replaced the thermostat. A board was replaced in the heater unit a few years ago (the old one burned out). This problem seemed to get worse after the board was replaced. We are experiencing colder than normal morning temperatures at this time, so the problem is being more evident. Once the outside temperature heats up, there is no problem.