Categorise by order frequency

Hi,

I'm trying to find a way to categorise customers by frequency of their orders; and then use this to flag when a customer drops off from their usual order pattern.

So: Customers that place orders on each consecutive month, that then do not place an order for 1 month are flagged.

Customers that order on alternating months, that then do not place an order for 2 months are flagged.

Customers that order once per quarter, that then do not place an order for one quarter are flagged.

I have measures that calculate days and months since last sale, and a measure that categorises them into an age bracket since their last sale. I just can't work out how to apply anything that measures the consistency.

Any Help would be greatly appreciated.