I forgot about the muddy-rust colored slime issue. As a precaution, I want you to go to the autoparts store and get Prestone's heavy duty coolant flush(not the 10min kind). Follow the directions except I want you to drive around with this stuff in the radiator for a week. Drain that crap, flush per the directions and then refill with 50/50 water and coolant mix. You could also take the car to your local quick lube and they likely will charge you about $100 to do the same flush I just told you about. We gotta get that radiator clean so the coolant can do it's job. It's probably several things combined that are creating this problem. You've got a fan that's not working correctly, the coolant mixture is probably out of balance, and you MAY have a faulty or damaged thermostat. In any case, overheating or "boiling" over isn't normal, running or not. The coolant will expand some when the engine isn't running due to the water pump being stationary. If the coolant temperature is already too high, let's say 210 plus, it doesn't have to rise that much to reach your coolant's boiling point. I'm not sure what thermostat is required in your car but it should be between 180-200 degrees, this is the temp at which the hot engine coolant will circulate through the radiator. So, like I said...clean radiator, good fan, good coolant, working thermostat...no more overheating...probably.

Oh, and let's hope you already have "good head gaskets"...