I recently had that almost exact thing happen on my little 4 banger Nissan Frontier! Heading up a moderate hill at high revs (unloaded, it's not my tow vehicle or anything), and then sudden and complete overheat into the red to the point of immediate concern. In my case I quickly turned on the heater and slowed down, and within 20 seconds the temp was normal again. I wrote it off as an air pocket, surmising that the change of flow pattern with the heater core now engaged freed up the bubble. To be safe I did change the hoses and put in a new T-stat after that happened, as it's a 2000 on all original parts.
