Engine shuts down

My van is a Campervan that when it seems to get to a certain temperature, especially when pulling mountainous roads, will lose power, sputter, then shut down. When I let is rest for 45 minutes to an hour it starts back up and continues to march. Lately, roads don't have to be too mountainous for this to happen. Temperature gauge on dash stays in the mid range the whole time, never going close to the hot zone. The engine is a 351c.