a couple of times you've mentioned that the aux fans kick in at high temp but you always say you switch it off then - what happens if you let it run? the high speed fans should cool it down then eventually switch off. they should already be on by the time you hit 110C. if you switch off then the heat soaked engine has nothing to draw off the heat and the temps will climb even higher and possibly cause the release by the pressure caps in the overflow tanks. if you have plain water in there, it will be easy to cause this as the boiling point is lower. i did see that you mention that coolant makes it bubble over more easily which is the opposite of what i expected. i would tend to suspect the themosensor that triggers the high speed fans. i believe they should come on before 110C at which point temps should start dropping rapidly...
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"