Without the plan and the query, there is not much we can do. Performance tuning is not about telepathy.

You could try SQL Sentry Plan Explorer which permits you anonymise a plan. But that only gives us the query plan, but not the query and nor the table definitions which we might ask for later on.

This article on my web site discusses parameter sniffing and how you can deal with it: http://www.sqlsentry.com/plan-explorer/sql-server-query-view.asp

But if your employer/client does not permit you share information in public forums, and you are not able to resolve the issue from articles like mine, your options for free help may be exhausted, and you may need to hire a consultant.

Note that the percentages you see are estimated costs, and may not reflect the actual costs.