if that's the only symptom, it probably isn't a BHG, but check for other BHG symptoms just in case (gurgling sound behind dash, overheating, oil looks like mud, etc). definitely change the thermostat, it's a $5 part that could melt your engine, I just chane mine every other year for the hell of it (make sure you get a good one tho, you can test in some hot water to make sure it opens). It could be your radiator cap. rad cap is designed to allow water out at a certain pressure and temperature, kinda like a thermostat. They wear out over time also and could just be letting excess coolant out of your radiator. i would start with a new rad cap and thermostat and see if that fixes it.