If your 91 crx is warmed up and you put your heat on it works for 1-2 minues and you have to shut it off for about 3-5 to get another 1-2 minutes of heat any ideas?

If your 91 crx is warmed up and you put your heat on it works for 1-2 minues and you have to shut it off for about 3-5 to get another 1-2 minutes of heat any ideas?
Alot of vehicles run too cool at idle to keep the water hot at the heater core especially in cold weather.