My first thought would be blocking by another resource, so I'd focus there. However, it's possible that a recompile event leads to differences in the execution plan. Capture it when it's slow as well as when it's fast and compare the two. Parameters aren't the only way to get differences in a plan. Hard coded values in batch commands use specific statistics like parameters. Local variables are sniffed just like parameters in a recompile. Any of these could lead to a difference in the plan.