Check the cache settings in the PowerPivot service application configuration pages to verify that data is not transitioning ...

Check the cache settings in the PowerPivot service application configuration pages to verify that data is not transitioning to an inactive state, followed by file deletion from the cache, too quickly. If the settings are not the issue, the server is under memory pressure, and you should consider adding more memory or an additional PowerPivot for SharePoint server to the farm. For more information about this rule, see http://go.microsoft.com/fwlink/?LinkID=398716