SCOM SQL query killing instance performance

Hi!

Since a few day, a new query running against our SCOM datawarehouse db is killing the performance of the SQL instance. The query run 1'000'000 times per hour for a total of 100'000'000 logical read per hour, and this continuously. The query is part of the following Procedure: OperationsManagerDW.dbo.fn_BuildInstancePropertyDelta

Any idea what's wrong ?
What could trigger this new behavior ? 

Rgds
/Chris

Waiting on statement:

RETURN
   (
   SELECT 1 AS Tag ,
      NULL AS Parent ,
      ISNULL(S1.PropertyGuid, S2.PropertyGuid) AS [Property!1!Guid] ,
      S1.PropertyValue AS [Property!1!NewValue!ELEMENT] ,
      S2.PropertyValue AS [Property!1!OldValue!ELEMENT]
   FROM
      (
      SELECT PropertyGuid = Set1.PropertyXml.value('@Guid' ,'nvarchar(256)') ,
         PropertyValue = Set1.PropertyXml.value('.' ,'nvarchar(max)')
      FROM @PropertyXml1.nodes('Root/Property') AS Set1(PropertyXml)
      ) AS S1
   FULL JOIN
      (
      SELECT PropertyGuid = Set1.PropertyXml.value('@Guid' ,'nvarchar(256)') ,
         PropertyValue = Set1.PropertyXml.value('.' ,'nvarchar(max)')
      FROM @PropertyXml2.nodes('Root/Property') AS Set1(PropertyXml)
      ) AS S2
   ON (S1.PropertyGuid = S2.PropertyGuid)
   WHERE (S1.PropertyValue <> S2.PropertyValue)
   OR ( (ISNULL(S1.PropertyValue, S2.PropertyValue) IS NOT NULL)
  AND ((S1.PropertyValue IS NULL)
   OR (S2.PropertyValue IS NULL)) ) FOR XML EXPLICIT
   )

*/