running hot

hi all, yet another fault on my r25. I think the thermostat is stuck closed. The temp gauge in the car sits on the middle line and when I rev the car I can see water going into the header tank, I don't loose any water at all, but the top hose is very very hot and the bottom hose is only Luke warm at best, the metal pipe going around to the thermo housing is also only Luke warm. I also get a misfire only when hot and the sparks are very white

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