Steve,
I know you dismissed it earlier, but have you checked your head torques? 100C just doesn't seem hot enough to be spewing coolant all over the place unless there is a massive pressure buildup inside somewhere for some reason or another. Could be cavitation, could be surface boiling in a water jacket, or it could be that you are blowing combustion gases right into the cooling system.
Just a thought.... last thing I ever thought of on my motor and it ended up solving all of my problems.
JP
