I hope you can figure it out. I have a similar issue with my 85' 380. If the outside temperature is above 70*F, and I am in town, it will go a little above what I am assuming is 100*C. When I get on the highway however, it lowers back down to a little about 80*C. In the winter, when it is below 45*F and I am on the highway, it goes down to about 55*C-60*C and never goes above 80*C, even in town. It really depends upon the outside temperature on how my reading is.

I have only owned this car since December and the warmest days we have had so far have been the lower 80s, so when summertime temperatures reach into the 90s, I am really going to be interested to see what happens then. I think it may be my clutch fan not working properly, but sometimes I can hear it roaring, but not when the needle is above 100*C. I think it may be the thermostat stuck open?

Good luck with yours!