Oh great, I'm never going to hear the end of this one either! Well good news is, your in the right place for information. Bad news is... I need some more information.
Feed? How much are you feeding and what is the age of these birds? Are they in a brooder? Are you feeding them 24/7?
Simple questions... but for a simply answer... if your not already doing this.... you need to start.
1) 24/7 feed for the first two weeks. Especially important if you do not plan to put them on pasture.
2) At two weeks they need to be put on pasture or fed a restricted diet. You have to remember, your cornish x rock chicks are equivalent to a 4 week old DP and it will not be long before they start resemble a full mature bird (weight wise). So best thing to do is to turn the lights off at night, let them rest. However if it's not warm enough your going to want to leave the lights on. If this is the case simply just take the feed away and give them water 24/7. Put the feeders back in the morning. You want to resemble a 12 hour feeding schedule... 12 hours full feed... 12 hours no feed. I find the best way to accomplish this, without any extra work, is to put them on the suns schedule. They eat when the sun comes up... and sleep when the sun goes down.
This is not a unnatural cycle... it's unnatural to feed any chicken 24/7. Even week old DP breeds should sleep at night and if reared by a hen, that's exactly what they do... all you have to do is mimic nature... the rest will fall into place.
3) If you don't have them out on pasture... make sure they have plenty of room. Keep the feeder on one end of the barn and the water on the other... Make them exercise. When they are outside they naturally will do this by chasing bugs and foraging for various vegetation. Their ability to forage depends on how quick you put them out on pasture, the younger the better.
4) Cross your fingers... these are the horror stories that people always talk about. The unfortunate thing is that obviously your birds were pushed too hard.... (not your fault!).... their little hearts are having a hard time keeping up which will cause them to "flip" The only way now to help them... is to slow their growth weight down...
5) Do you have any pictures? This would help determine also what could be wrong. I can tell a lot by just looking at the bird... So snap a couple quick photos....
Good luck... keep us updated...