I have a 98 OBW with bad head gaskets. It was doing just what you described. I could drive for about 100 miles on the highway it would push the coolant out to the overflow and not pull it back to the engine. If I stopped let it cool and top off the radiator it would be good for another 100 miles. Check what the others said and watch your over flow tank, if it keeps filling up and draining the radiator sounds like the dreaded HG problem. This generation motor has an internal leak that won't drip outside the engine so can be tricky to diagnose, I tried all the cheap fixes (thermostat, fans running, etc.) but now the engine block is sitting on a table, the heads getting checked for cracks and I'm waiting for a bunch of parts.