P1290 FORD - replaced cylinder head temp. sensor, still showing P1290 code

Question

check engine light on, check gauges appears on odometer, temp gauge bottoms out, ac blows warm, then everything goes back to normal briefly, then does it again. this cycle continues as i drive. code reader produces P1290. i replaced the cylinder head temp. sensor and it appeared to be ok for about 10 miles then started doing it again. Truck has 204k miles. any solutions?