I've been through the whole Beta programme with iOS 7 and autocorrect only seems to go crazy for me when I'm experiencing bouts of keyboard lag.
Others on the forum are insistent that, "Reset All Settings" cures the problem and although they are trying to help, it doesn't and is merely a colossal waste of time.
The issue has improved quite a lot since the early Betas, showing that Apple is addressing it, but it's not fixed yet.