After getting a firm grasp on the engine operating ranges and having followed Yo-Yo's guidelines on engine operation, I didn't have a seized engine in more than 20 hours of flight time, in the whole range of operating modes, including WEP bursts.
Most of the time my rads are set to automatic, for low speed, I might open them up, but 90% of the time they are on auto. No problems.
It might be worthwhile to analyze your actions and see where the fatigue builds up...