Heater cold when idling

In my '94 Jeep GC the heater blows cold air when idling (like at a stop light or in a parking lot). If I put it in neutral and rev the engine, hot air comes out. Once I start driving (even slight acceleration), it gets hot.
I had the radiator replaced last year and that's when I noticed the problem. I hear gurgling like there are air bubbles, but radiator and reservoir levels look right. Please let me know what might be causing this.
Thank you!