i have seen s2,s get hot with a low co at idle but the gauge was rising aswell,i,d say its just a simple case of a crack in your header tank finally giving way.i take it that its a plastic type.the standard plastic cozzy tanks do this as well and you can actually see the "i,m just about to burst fracture lines on the tank surface" .
try a new tank or better still upgrade to an alloy or stainless steel one