Validation of viewstate MAC failed

Question

i used this line <machineKey validationKey=....> in web config for this 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. "

 

But Again same error is repeating after 2 days. 

 

Any one give suggestion