At the beginning of this month I began to receive a ton of leads coming from this one ad group, content only. I made no changes to this ad group for fear of breaking whatever it was that was working.
After a few weeks into the month, there were certain days during the week (not in a pattern) that I won't receive any leads at all. Then, all of a sudden it will pick up again, and drop off again on random days. What is going on?
Lots of things could be happening. Sites that run your ads could be having traffic ups and downs. Competing ads could have budgets that come and go. Run the Site Placement report for each day for that ad group, and take a look at the sites where your ads are showing, and see if you can find some patterns specific to the sites.