Low Coolant. There is a leak somewhere. If you don't see any in the engine compartment, then it is in the motor (Fingers crossed that it is not).
As the pump impeller spins more slowly under lower RPMs, an air bubble propagates inside the heatercore as it is normally on a bypass line and on the higher side of the loop. Under throttle, higher RPMs restore flow through the heater core to overcome the air bubble. It will also suck the overflow reservoir dry.
Adding coolant will temporarily solve the problem, but the leak must be found. Look for sticky residue when the motor is cool. Or if it has totally evaporated the glycol, look for a white residue near hose connections.
First, I would direct your attention to the radiator cap, and go from there.
'09 PSM G8 GT - OPG Intake, DOD-Delete w/ Stage 1C 218/229 .600 Lift 117.5LSA, Kooks 1 7/8 Mids, Solo Axle-Back w/ X-Pipe, SuperPro Bushings (Front and Rear). BMR Sway Bars (Red)
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