Not sure. My 85' is erratic as well. I have only owned mine since December, but we have had a couple of lower 80* days since then. The warmest day, I noticed that my needle went to the line between 80*C and 120*C - I am assuming this line represents 100*C, since it is perfectly in the middle there. This was in slow city driving. That was the highest I noticed then I hit the highway and it went back down to 80*C. I replaced my cap, as it was not sealing properly, but that did not help. I know that when it it below 45*F outside, and I am on the highway, many times the needle will stay around 55*C-60*C. I am thinking perhaps my thermostat is at fault - maybe your problem as well. I would like to change mine, but I am sure like everything else with Mercedes, there is something difficult about changing it.