Okay, people.. this is how it works on the 737..
Fwd and Aft doors open, cold air rushes in and the thermostat senser in the fwd cabin registers it being COLD... then the air system pumps in HOT air... simple... it's like a cat chasing its tail...
Only way to stop it is .. shut the front door... harder to board passengers that way... or get the fo to turn the temp controller down....