I have same bike, same issue. I jetted mine down to a 112 main, dropped needle one, live at 7,000' in mtns.
Mine will only overheat when sustained WOT, at 3/4 throttle, it will run all day not puking coolant.
I figure the main jet is too small for sustained WOT use, and it leans out. My plug will read really lean after sustained WFO.
So, if you are still stock jetted with a 118 main, try going to a 120 or 122 and see if this stops.
My champ class buddy says they jet the trials bikes really lean on the main, so it will clean out and pull hard in section riding, but that same jetting is too lean for sustained riding on the main....