If you weren't watching it and filled the coolant up to the max level when the engine was cold, it would do that once it got warm and the fluid expanded. But if you're topping up every three months or so, you've got a small leak in the system somewhere. Crawl under it and check the hose at the bottom of the radiator. I flushed the radiator once on my 97 GLT, filled up with new coolant, drove it, parked it, and had a tiny puddle of green fluid on my garage floor the next day, which I noticed when I pulled it out. I really tightened the hose clamp when I reconnected the hose, but not hard enough. A lot of pressure builds in that system when the engine is warm, and it will force it's way past your hose clamps if you don't really tighten them down. Always double and triple check for a week after you change coolant just to make sure you have no hose leaks. Keep in mind though, that the radiator hose connector is plastic, so don't get too heavy handed with the screw driver. Dick