My furnace kicks in and shuts off within seconds. Sometimes it does this several time in a row. Sometimes runs perfectly normal. Thermostat fluctuates by 3 degrees as I'm standing there looking at it. Any ideas?

Model and brand of furnace. Also get the brand and part number off the gas valve. The last part is important because there are two different valves under recall repair. One is from years ago and the other from two tears ago.