check all jubilee clips on every water pipe for splits ,not tightened ,brittle etc as i had this problem on my r5 gt turbo
baffled me for months turned out the water pipes around the water pump had gone brittle enough to draw in air but not leak bizarrely lol......
so basically when water pump was working hard it was drawing loads of air in ,causing airlocks then overheating ffs lol
very similar sounding try it regards and it costs fook all too

one way of doing it would be to get a old header tank cap and modify it to fit a valve in then pressurize the water system until you find the weak spot /problem area