Nikolaus, I agree with you I think it is a heat issue too. Do you have a fan blowing directly on your plants? I am also think that you may be over watering them, I know the top of the soil dries out but it may not be only be dry for the first inch or two of the soil and wet beneath. Home Depot and Walmart have a cheap wand type device that checks how wet the soil is towards the bottom of you pots. I think it costs around $5. Are you on a 24/7 light cycle?