I have the same exact problem bud. Its definitely not nutrient burn because this is only seems to be happening on the middle/top fan leaves for me. Out of about 40 plants it seems to be completely random on which it affects. I'm pretty sure it's not a deficiency either because I give my babies a health feeding once a week alternating with a plain water/molasses solution. I have researched it a bit and I have come up with a couple ideas. One could be very hard tap water, which I know I have because my tap water has a ph of about 8.5. The dissolved salts and chlorine in the water is very unbeneficial. The other is that it is actually light overdose. I have moved my 1000w about 6 inches up a couple days ago and so far it hasn't gotten any worse. If you find out exactly what it is definitely let me know for shure. Hope I could be of some assistance.