I had over heating issues when 1st bought my car, for me it was the radiator which was fucked, if the rad is fuckered the coolant will not circulate around the engine properly and give you the symptoms you are describing, rad fans not kicking in is one of them, if you take the fan switch plug off and bridge out the circuit with a hair clip or paper clip and the fans start to blow that tells you that everything from the car (circuit) is fine, this usually means that the rad is blocked and the coolant is not circulating correctly, fine-ish when you are driving and air is blowing through the engine bay, but when caught in traffic the fans don't blow and the engine over heats,

I'm a little concerned that you are saying coolant is blowing out of the header tank cap, this can indicate head gasket failure

as for the thermostat, 4x4 stats are notoriously unreliable, the way to check if you have problems with the stat is to get the car out on a run, sit at 100mph and if the temp gauge starts to creep up the stat is fuckered, I have always drilled a small 3 mm hole in stats that I have fitted to my car, this way if the stat does jam shut you will have some circulation of the coolant,


Luciano