4.0 Overheating Problems

Two months ago I replaced both head gaskets and the thermostat. The water pump was fine so left that and the heads didn't need skimming. It was bled correctly, rad was hot all over too. It was fine for a few weeks and then started to get to hot again. Bled the system a second time and all was well.

Now for the past week she is overheating within 4 miles and spitting out coolant once the temp gauge gets past 3/4. This is where it gets a bit strange. The heater is blowing hot and there is water in the pipes so the pump must be working, I put an emissions tester over the header tank, there is lots of un burnt fuel but no exhaust gasses coming through, the radiator is now only warm at the top, obviously this will cause it to overheat but surely not within ten minutes idling from cold. There is no water in the oil.

Due to it overheating so quickly I'm suspecting something more serious is wrong.

Anyone had a cracked block or head on their v8 and what were the symptoms like?

With a dropped liner it will overheat in about 10 mins at idle,mine did, but if i drove it it would drive for about 8-10 miles before the red light came on.i would remove the thermostat and try running it without,when was the rad last changed as they are known for silting up,i take it the small pipe from the top of the rad to expansion tank is clear? do you get a squirt of water into the expansion tank from it,dont write the engine off just yet.