Atz1

Hello out there! I own a 2003 Ford Ranger. This unusual problem popped up. When using my heater, rpm's drop every 5 seconds or so and I hear a click under the dash. Now, this only happens when I attempt to use the heater for two areas, example feet/dash, or feet/defrost. It also happens when I switch to straight defrost. If I use only the panel setting alone, or the under dash setting by itself, this problem does not occur. any ideas out there? Thank you for any assistance. Jim