Help!!! 2.4 puma overheating issues

Hi guys new to the landrover scene here total novice and I've recently purchased one a 2010 110 utility 2.4 puma and I've had nothing but problems from day 1 the issue started with slight loss of power a few seconds after it would be fine and all power would be restored after a few times of this happening the temp gauge would all rise with the loss of power and its building quite alot of pressure that much pressure it actually split the header tank/coolant expansion tankin two halfs and put quite a large dent in my bonnet but only happens on occasions some time it can last 50 100 miles no problems no smoke at any time hot or cold so I've taking it to my local LR specialists and they had it on diagnostics with no fault codes at all the did a gas test on the header tank and said ether the cylinder head is gone or the water cooled egr valve has any would on here had any similar problems and managed to solve them I've replaced the radiator and header tank water pump seems OK aswell tia any help is appreciated and welcome

After fitting a Elton cooler my coolant temperature came down between 2 and 3 ° c, the head temperature stayed between 95 to 98°C.I use a Scangauge to measure both temperatures (I think the ultragauge display one temperature for the coolant which is most probably an average of the coolant and head temperature).

It may be worthwhile to try and measure the coolant temperature with an analog gauge and compare it to the Ultragauge reading.

For interest sake, my coolant temperature sits between 85 and 87°C and head temperature is between 95 and 98°C.(That is on the N17 between Devon and Delmas offramp).The flattest straight of about 20km I know of.On the slightest uphill the head temp will climb to 98 or even a 100°C and the coolant will drop by a degree or two (83°).