For the last couple of years about every 3 or 4 months often when towing a trailer (which would probably be equivalent to giving it a thrashing) the car (4x4 saph cos) would overheat - particularly when stopped in traffic after a long run on the open road. There was always plenty of coolant - in fact it would be pushing it out of the top of the header bottle. After quite a while on the side of the road it would suddenly suck all the coolamt back into the engine and I would fill it up and away I would go and it would be alright for another couple of months. I suspected the thermostat was intermittantly not opening so I replaced the stat with a new 82deg one. went away for xmas (summer here in NZ) and the car ran nice and cool all the way up north and when I stopped for gas the same thing happened. After holiday with no probs headed home (3 hour drive) made it home and in the driveway temp rose and what looked like all the coolant it would hold (well heaps anyway) poured out on the driveway. Left the car overnight and the next day the level in the header was still on Max. Started car expecting it to drop away once pumping around but stayed up. Now as it heats up coolant rises in header to overflowing and when I open bleed at thermostat nothing comes out until I open header tank lid then water flows out of screw hole. BUT I have still not been able to put in the amount of coolant that obviously leaked out in the first place and it now has a constant overheating issue. Obviously head gasket is one suspect but why so intermittant up to now and why is the header still full if loosing water this way. Fans not coming on (switch) is another. I do have an override fitted and this did bring temp down initially this morning but further on did not. (remembering I suspect a lack of water in there somewhere) Does the cap cause issues - blocked vent or similar. Sorry for such a long winded story but I am unsure where to start and we don't have the expert tuners here in New Zealand that you enjoy over there. Thanks David