P38 2.5 diesel, water in oil, head gasket again??!

Engine problem on P38 2.5 diesel 1995:
Ran well since bought 2 years ago. After a short journey on the motorway the water expansion pipe burst, most of the engine oil had entered the water system.
Removed cylinder head, one centre head bolt loose -the thread was stripped. Had a helicoil inserted, used new head bolts and replaced head with another second hand head because of cracks showing between the valves and injectors.
It then ran ok for a week apart from poor starting when hot (started ok cold). After a 2 hour run on motorway it suddenly overheated when pulling off motorway, this time water had got into the oil. I have checked the block and head for signs of warping, both seem ok. No threads are stripped and there are no visible signs of where it has been leaking across the head gasket.
I feel it may be a waste of time putting another head gasket on, has anyone got any suggestions?

Hi I have a Range Rover p38 2.5 dse I Have had that exact same problem head gasket went so took off the head then found out the head was cracked so got a second hand one then put it all back to gather it lasted about one week and oil in the water again and the head gasket went again! So took the head back off again to see if the block was warped but it was fine. So long story short found out it was the radiator that was the problem and is been fine since! Hope this helps!