But the cylinder head is higher than the heater rad by a long way. By that measure the head would be full of air and have melted by now?
Plenty of 335i owners have had their water pumps replaced, thus the system completely drained, and the system re-filled on a flat ramp no problems?
If the OPs started working on a tilt, under this theory that would mean the air had been expelled, so why did in stop working when put flat again?
Worth a try though......anything to get warm!