Overheating issues

[Jaiden Crook]

Hey all. I recently purchased a 2008 Silverado 1500 4.3L V6 model truck. I have noticed on incline and at idle for long periods of time it starts to overheat. It never overheated while moving more than 20 mph. Rpm doesn't seem to affect it. Idling at a higher rpm still causes it to overheat. If I turn the heater on, it immediately cools down to operation temp. I don't leak coolant at all. I can hear a slight gurgling coming from the heater core area. I assume from lack of flow. 

 

I have changed the heater core, thermostat, and did a full flush and refill of dexcool. My next guess is water pump but I'm wondering if anyone else has had this issue and can point me in the right direction. Any clues would be great! Thanks