2003 Honda CRV tempature gauge issue and inconsistent heati

for a while now when the heat is on the car heats fine when the car is running and then it blows cold air when the car is stopped (i.e. at a red light, in heavy traffic) also I have noticed that the temperature gauge while in traffic rapidly rises to the top of the gauge and I am fearful the engine could be about to overheat, usually in such situations with other vehicles turning on the heat (even in summer tends to lessen the temperature gauge to a lower level but that does not occur in this situation