recently while towing a 2 ton van the Nav started to run towards the red while going uphill. as soon as you start downhill it cools quickly. thermostat has been replaced, pressure tested and radiator core cleaned. The Nissan dealer has had one go at it along with a Diesel mech that did the thermostat. Seems to run o.k. when not towing the van, any ideas as I am a completely useless mechanic. It has been chipped, hotside pipe done and EGR block. Performes exactly the same but with less power if the chip is blocked off also.