Hot, stagnant air and/or overfertilizing could be the cause. Even though ambient temps are around 70 doesn't mean it's only 70ish at the bulb. Is there a fan on 'em all the time?

When my temps rise, so do the leaves.
Usually doesn't affect the serrations though, which leads me to believe possible (minor) overnute also.

Keep in mind that these are just guesses, as I don't know your other parameters and haven't seen 'em. :thumbsup: