Thank you for the link -- a good explanation of parameter sniffing. Perhaps you're on to something with parameter sniffing. My Confio tool (Ignite) did say that the execution plan had changed for this query -- though when I compare the two execution plans they look exactly the same to me (as viewed in SQL Sentry's Plan Explorer). There has been a schema change -- a bit field was added to one of the tables used in the query, though that particular new field is not used anywhere in this particular query.

The query brings back customers for open sales orders. I ran it individually for each of our franchises and the number of records returned didn't vary all that much (approx. 10 - 100 records per run).

Rob