On arrival, furnace is lit and operating. Technician furnace at thermostat. Observed furnace shut down and start up. Replaced 16 x 25 by one filter that was quite dirty. Furnace/system is extremely old and it is possible that a dirty evaporator coil is restricting airflow enough to cause the furnace to over temperature especially with a dirty filter. Furnace is operating at this time, although replacement would be recommended just because of age and condition.