Issue when Retrieving Data

I receive the following message when trying to retreive data from my page tigergps.org/InventorySearch.aspx. This error occurs only occasionally, and seemingly at random times. It seems like it must be a hosting error, since nothing changes within the code in between errors. Any suggestions?

Error:

Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Web.HttpException: Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.