@Spencer Rivers check campaign settings if your goal is set to campaign specific goal or account default goal. By default it would be set to account default goal, where page views and other metrics would be considered as conversion. If it is in account default try changing it to campaign specific goal and add sign up or form submission as the conversion goal. It would reduce the mismatch and in the conversions tab check other than your main conversion action others are in secondary, sometimes because of this the values could be inflated.

Along with what @Steve Cox said, could you please check the search term list and see if we are getting more clicks for any irrelevant terms and compare the data of the top performing keywords which was performing good and during the period of performance drop & see if we are getting clicks without conversions. Try replacing by the keywords with similar keywords.