the same thing is happening to me as to Peter Krogh,  after replacing the thermostat and bleeding the system for two entire days, same thing happening........level light will blink eventually, almost always at highway speeds, expansion tank drains to overflow and pisses all over the back of my van, tried changing cap, same shit happens, no leaks found anywhere, continue to bleed using several different methods described by ya'll, now today when going to test her again, right side head gasket GONE, limp her to this same shop that recommended initially (get this) try powerwashing the radiator fins,  does any of this make sense to ya'll???
would the gasket give because of the coolant mystery???It never really got to hot,  or,  was the rising coolant due to the bad gasket?  there was no leaking prior to today???????

paul
91 GL et. al.