As Erland Sommarskog says, DBCC FREEPROCCACHE will indeed clear the plan in question and all others; be careful with the use of it and not good on production and from you're first post you're on production?

It's looking somewhat like it's a stats issue so maybe take a look at the execution plan and see how many rows are being estimated compared to how many rows are actually returned, big differences could indicate outdated stats. In a previous post Erland mention SQL Senty Plan Explorer, go download a copy (it's free) and have a go. It really is a great tool and it will put all the stats info into a table for you highlighting where estimates differ from actual values.

Mart