Poppet valve can cause overheat at idle, and cured at higher rpm if it is stuck partially open by debris. Higher rpm compensates and overcomes low rpm water volume
Have personally experienced it.
After checking poppet first,(it's easier), I would check water pump next.
The housing and especially the base plate has to be perfect, no deformed plastic and no grooves or galling at all.
Grew up on Tawakoni and Texoma before hybrids and stripers existed on either.