Choir question

I am teaching the "Amen" section of Worthy is the Lamb in Messiah. This is definitely the hardest chorus for the students to learn. Each voice section knows their part, but putting it together SATB is a completely story! I have tried count singing and that helps just a little. But even when they get it somewhat correct on count singing, when we switch back to words, the whole thing falls apart again!!! The kids are so good, and they are really trying, its not a question of being lazy, its a matter of what strategy will work the best. I tried having them sing on sticcato, and that helped them to not slow down, but it didn't help "hold their own part" when they are all singing together. I have tried sectionals, and that helped them learn their part individually but it didn't do anything to help the choir as a whole. I am really stuck, and I was wondering if anybody had any suggestions. My co-op says not to worry about it and it always comes together before the concert, but that doesn't make me feel any better right now!!!