From time to time my 500 E starts hunting / surging between 600 and 1200 rpms with very sudden "attacks". When this situation occurs, I notice at the same time, that the temperature raises to about 100 C. When putting the gear in D, the temperature eventually settles back to its normal operating temperature, and the surging seems to go absent.
And then again, it can be several days, even weeks, before it happens again.
The temperature is spot on 82-85 C, if the issue is not present. I do however live in a country with very cold temperatures (September to April is below 0 C), plus I just bought the car, and therefore haven't tried driving the car in the high 20C 's and 30's.