Check ph of run off. if using GH Just right xtra soil i think that stuff is ph'd. i killed a set of seedlings in that stuff. tested run off water and ph was @ 7 for run off. and i ph'd the water to 6.5 before watering (because thats what i do for Happy Frog). i think that was my problem. hope that helps. and now i read it again and you found a heat problem lol. did it fix the problem?
Regarding the GO box how often are you feeding. i just started using it as FEED, FEED, WATER. Do you take breaks like that to just give straight water?