[olddog]

When it is hot and you shut it off, is there any gurgling? Is so you either have an air pocket or steam pockets. If there is no air pockets and steam forms, it will build enough pressure to push water/steam out the overflow.

If there are no air pockets and temp is going high at low speeds, you do not have enough air going through the radiator. Fan, shrouding, air bypassing around the radiator, etc.