Validation of viewstate MAC failed.

Hi
i am doing a web application in asp.net with vb. the problem is that when hold the website at least 10 mint it shows an error
 
Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that 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.
 
Thanks
Afsal