one more thing you might want to check after the fact. my 95 dx blew a head gasket too. it would drive fine all day out on the highway just like yours. if i shut it off and restarted it before it had completely cooled off it would really spew out some antifreeze from the overflow. at first i didn't realize what was going on, but then i understood because the head gasket was blown, the cooling system was getting pressurized by exhaust gas when i started it. after it was running for a minute it was ok. strange. all this high pressure also caused my heater core to blow, but i didn't know until later. actually replacing the head gasket and the radiator was easier than the heater core. might want to check around under your floor mats to see if you've got some leaks.