Overheating Issue

[Usmc1028]

I'm going to try to explain this the best I can. I have a strange overheating issue. It only seems to overheat when my heater is off, and when the gauge does start to go up, right when I turn my heater on its goes right back to normal. When its on, it does just fine, never goes past center. Things ive already replaced: thermostat, water pump, expansion tank, lower and upper hoes, cap, bleed screw, transmission thermostat, drain plug. Another strange thing, I can leave the car sitting at idle, and it wont over heat, it only happens when i start to drive it and even when i start to drive it, takes about 15-20 miles before the temp starts to go up. I've bleed the system multiple times, no luck. Any clue what it could be? Heater core? Water valve? Temp Sensor?

Also i currently do not have any A/C pressure in the lines right now due to a motor swap. Could that have anything to do with it, if im correct, one of the A/C lines are connected to the heater core.

2006 BMW X5 3.0