You may have plan cache bloat. Glenn Alan Berry has some Diagnostic scripts for 2008 that can show you single use plans and lots of other stuff. I would check for those first, and decide how you want to proceed. If you have lots of dynamic sql coming in (from ORM possibly??), you can deal with that with a variety of approaches such as optimize for ad hoc workloads setting or even dbcc freeproccache. I have clients I had to set up jobs to do that on sql 2005 as frequently as every 30 minutes to aleviate memory pressure from single user ad hoc plans.