I've been religiously reading several S80 forums since 2001, and, other than the well-known cooling fan failures, have not often seen complaints of overheating.

If you cooling fan is the cause, the car will tend to overheat at idle and low forward speeds, but recover on the highway, where there is ample airflow through the radiator with the help of a fan.

If you have an internal coolant-to-cylinder leak, the symptoms will reverse: overheating at high RPMs, and recovery when you slow down.