I have had the exact same issues happen a few times (4-5 times) in the tight woods, and I specifically checked to make sure the radiator was topped off before each of those rides, and after the ride all the anti-freeze had disappeared from the overflow, and radiator looked empty too. Mine is an 03 220 as well. I tried changing to engine ice, and upon refilling the system I used the air screw to bleed the system. Still have the same problems. As of right now, I check the radiator before every ride, and even during rides and enduro's. If low, I refill with my camel back until the race is over. Then it is home to flush the system again and refill. I am thinking about taking the overflow off all together, as I read a few people say this solved their problem. Until I hear another solution, I will be just refilling and keeping an eye on it. I don't know what else might be the problem. My oil isn't milky, and the bike doesn't have any other problems. It seems to be running good.