Can you monitor execution plans (via extended events), get a plan from a fast execution and a plan from a slow execution?
Can you monitor/record wait stats per query during its execution (poll sys.dm_os_waiting_tasks) or extended events and see what the query is waiting on when it executes fast and what it's waiting on when it executes slow?
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability