Temperature gauge & interior heater fault

[Sibbo]

We have a 2011 A3 2.0tdi black edition, DSG, 170bhp. When stationary temperature gauge eventually gets to 90 degrees & cabin can gets hot but once your driving, the gauge drops to about 60 degrees & the cabin looses heat. Thermostat replaced, with genuine part, system bled, once bled, stayed hot & gauge correct, but left for a few hours, fault returns. Had the system rebled & again gauge & heater perfect, until left again, fault returns. Heater matrix removed from behind the dash & flushed through, found not blocked. Refitted, ok when stationary, but start driving & gauge drops again along with the heat. 

anyone had this fault, please help!