The oil burner will kick in when calling for heat but will only run for about 2.5 minutes then kick off while the fan motor continues to run (not burner fan), then the burner kicks in again. This happens up to 4 times during the cycle period. What are the possible causes of this cycling on and off the burner? I've changed the thermostat which was needed. New transformer, there is soot on the view portal, flue has soot, cad cell tests fine, primary control is making a hum.
[email protected]

This sounds like the heat exchanger is getting too hot. It could be cutting out the burner on high limit. It is possible the heat exchanger or your a/c coil if you have one could be blocked. Do you get any heat out of your registers when the fan is running. Are your cold air returns all open and free of obstacles? What is the make and model of furnace?

If you are seeing soot, it's past time for a cleaning & service.