Yeah I remember seeing that picture when I was browsing here a while ago. That's exactly what I've seen from (very) mild heat stress.
Probably with high humidity and high temperatures your plants normal ability to cool itself down(evaporation) is not able to occur efficiently due to the high humidity.(more water in the air, less water evaporates) With luck the problem will go away if you lower the humidity, but I would not count on that being enough. I'm sure genetics also have a fair amount of influence on what temperatures your plant will be able to tolerate.
You either need to turn up the AC, turn down the light, or move it out of that closet with the conditions you describe. With you temperature in check that light should be able to sit 12" or closer(I've had a well cooled 1000 watt at 12" with no adverse effects) If you can pump the heat from the light outside of the grow area that is by far the most ideal solution.