This furnace was installed in 1985 as part of construction of a new R 2000 home. It still heats the house fine. However at startup the burners are on for only about 20 seconds and after about 3 seconds the burners are on again for only about 6 seconds and for the rest of the heating cycle the burners are on for about 2 seconds the off for about 3 seconds.
If I squeeze tightly the white plastic tube with the rubber band around it (see image), the burners stay on as one would expect. We have cleaned the flame detector.