Are you still using fluros on them? And if you are, do you have a fan blowing around or on them?
Usually if you dont have any sort of a wind effect, stems get weak, and make the plant keel.
Also, the droopy-ness....if it was a under water or over water, that plant would be pretty close to dead right now...wouldnt have made it 3 feet if he wasnt really that on top of watering.
Its probably a defiency...I would go with what everyone else said to do.
I bet once you get them under that HPS, that they are gonna straighten right up, because of the lumen power of the HPS....but your gonna hafta correct that defiency first...
Also, for bigger stems....cant you just up the nitrogen intake?
My 2 cents...peace.