I swim 12 - 15 open water races a year. No two are ever the same. That is one thing that I love about ow. Because of the differing conditions I have found that unless a new skill is really, really, really imprinted in my brain and muscles it will fall apart at least once in a race. It's hard to keep focused when there is all that thrashing going on around you at the beginning. If you are falling apart near the start of the race begin away from the crowd where it is quieter and easier to concentrate on your stroke instead of the madness around you.
While you are swimming by yourself (in the pool or lake) find a focus that gets your stroke back when it doesn't feel right. For me it is connecting my hip drive to my spearing. That way when you have your "disconnect" you have a more singular focus rather than your whole stroke.