I keep my lights a minimum of 24 inches away, and sometimes certain strains (a perfect example is Sweet Seeds Mahan Ram) are more sensitive and more distance is needed. I'm at 30 inches with my 1000 watt HP horizontal shields. It is a heat problem- that's a no brainer- raise your lights a minimum of 6 inches or more- and it will improve.
Also, too many fans along with the heat from the light will dry the leaves out eventually and this can contribute to your issues. It's all a fine line sometimes.....
So the fact that the newest growth shows no problems (the newest growth being, of course, closer to the light than any other part of the plant) does not enter into this? Do the tips have more moisture in them, allowing them to remain green? It sounds like that would have to be the case in order for the leaves one and two sets down from new growth to be the ones that are showing signs, if this is truly a heat issue.
Funny, my Afghans sit a lot closer to the light but show no such signs, and I would have thought a sativa dominant strain would like higher temps than my other plants, all of which are indica dominant.
I HAVE moved the plants down a bit and in about three weeks the Afghan is coming out of that tent so I will have more space to, say, start pulling the longer flowering diesel's top cola (or what there will so far be of a cola) toward the sides of the tent, encouraging distance from the light and interior growth.
It is not LST per se and I doubt that I will bend, when it is all done (over the course of a week or more), more than thirty or forty degrees and I may have to wrap it around a bit. It will share the 4x4 Grow Lab with a White Widow for all except the last week or so if its own flowering cycle.
I have read that training is best finished by week three of flowering but should this small and gradual bending be a problem? It seems called for if this plant is having issues already. This lady should have another seven weeks of flowering left!
Most importantly, though, and as I asked above: does the fact that the newest growth looks pristine bear on this at all? Does it take time for heat symptoms to manifest?