A little help

hi all, I've got a 2010 defender 90 tdci and having a little trouble with it lately, it's using coolant and the heater is blowing hot and cold. If it's been driven for a long time the temperature gauge will shoot you then will drop back down. I've replaced the thermostat, water pump, heater valve, water cap. The radiator and water pipes have been blown then putting coolant back in and bleeding numerous times. I've also had it pressure tested and the head tested and failed none of them. Just need some help as running out of ideas unless I start spending a lot of money on it.