somewhere your frog lost its water tight ass, try a coolant pressure test. look for pin hole sized leaks in the tubes and in the rad itself shoot for 15psi. as you know coolant expands when it gets hot, the vent and breather on the rad cap acts like a siphon when it cools and the water retracts it will suction in from the coolant over flow and level off the system.
poor cap, or bad coolant hoses is my guess, or perhaps main coolant lines have a bad seat due to build up of corrosion.
