The water pump will work or it won't - there is no intermittent.
It could simply have been the weather conditions and being in a bit of "bad" air following the other cars up a long climb. Mind you, I've been up and down Tamborine and Nebo/Glorious heaps of times in all weather behind other cars with no issues at all.
If your engine driven fan (if an auto) and your electric fan are working and your coolant level is OK when cold then it was probably just a one off. I think you said the coolant level was OK and that the coolant was changed not so long ago anyway. Maybe it just needed to bleed out an air bubble from the last time it was flushed.
Just keep a close eye on it and see how it goes. Check the fans work properly (check their operation from time to time) and that your serpentine belt isn't slipping - which it shouldn't if your belt tensioners are working OK.