Maybe you've got some weird lubricant breakdown going on inside the freewheel at those temperatures. You could put the freewheel through a solvent bath (mineral spirits or similar) and see if the problem remains. I'm sure it would be fine without any lube for a ride or two as long as it isn't also gritty. If the problem still occurs it's probably something else and you can relubricate it and continue on the search.