Looks like the heat is affecting them right enough, is it only at the top of the plant? Have you been foliar feeding up there, and if so, check the strength, and consider when the optimum time is to use it and ensure the droplets don't sit on the leaves and act as a magnifying glass in bright light, add a wetting agent to your foliar feeds of course to aid quicker absorption, low light level application, before or after light period, this is awkward in a grow room during flower I know, either pitch black, or dazzling light, although I see on e bay a gadget one can use to see safely in the dark a bit, a green emitting light which apparently doesn't affect the dark period in an adverse way, allows maintenance at an awkward time. Spider mites are a nasty problem if it is, you can actually see the little buggers through a decent magnifier glass, little whie dots moving around!! also there will be some very fine web around the leaves, especially underneath. There are treatments for spider mite, some organic, like neem oil, or of course much harsher chemicals, there are a lot of claims out there, so if you have to try something make sure that it has at least good write ups, many of these solutions have mixed results, some are reported as being ineffective against spider mite, as I said they can be a nasty problem, in your case best hope is to control them as much as you can and race to the cropping post, however, it is unlikely to be them imho. As your heat is an issue, s you mentioned, and it is a recent developement, it is likely that this is specifically relevant.