Hello and welcome to Justanswer!
I see that your question has been up for an extended time without a response and while although this vehicle isn't in my area of greatest expertise I would like to try and help.
I have to wonder if there is still air trapped in the cooling system. If there is air trapped in the system because it wasn't properly bled then it can cause boiling because the air will compress and system can't properly pressurize. The system being under pressure (controlled by the radiator cap) raises the boiling point of the coolant about 3 degrees for every PSI.
With the engine cool remove the radiator cap and add coolant if it's low. Start and run the engine until it's at full operating temperature and no more bubbles are coming from the coolant. If bubbles continue to come and never stop then you still have a combustion leak into the system.
Once it is at operating temperature and no more air comes out then install the radiator cap and see how it does.
If the coolant continues to drop with no external leaks, you have continued overheating or you continually have air coming from the cooling system then you still have a combustion leak.
thankyou for your response. I have done as you have suggested and am monitoring coolant level etc. the answer I was after was if there was any other known faults (like the cracking head problem) with these engines, especially ones that could cause this problem. hopefully not and it is just air trapped in the system