water temp over heating

ok i have an 01 century with a 500 hp detroit .....about once every six months or so the water temp starts jumping obv not an accurate reading as the oil temp is steady this time when i pulled off to look at it the cel went off and it shut down in protect mode...usually i disconect the sensor and hear the air release in the coolant system and it resets no prob again for six months or so

i took it into a dealership a few years ago they checked the sensor gauges etc couldnt find the problem again they blamed it on the ecm but that didn;t make sense to me any other places to look