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Other than the end of the month, there's no obvious reason why this should be so in our market, which leads me to conclude that this must be the "fault" of Google. In particular if we have an outstanding day of clicks, it is almost always followed be an equally underwhelming day.
Just to help me clarify one more piece of the puzzle - does anyone else see this apparent acceleration and throttling through the month?
I remember one account I worked with where the patterns didn't make any sense until laid over a calendar and investigated. The conversions were highest the first weekend after the 2nd payday in the month. Turns out that's pretty common for high-ticket items and the fact the first paycheck goes to rent/mortgage and the 2nd to disposable income.
There might be a pattern in there somewhere that's worth finding.