JalapenoNotch0

Most likely it is your heater core. The "fog" coming out of your vents is the "green fog of death" that indicates a hole in your heater core and the misery involved in fixing it. The flush you used probably removed some junk in it uncovering the small hole. If it is overheating you may have a bubble in your cooling system. I'd probably guess it's not the guage. I've had a similar problem minus the heater core. Try bypassing the heater core like they said above and see what happens.