Validation of viewstate MAC failed error

Question

Hi:

I've been encountering the following error :

Server Error in '/' Application.

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 algorithmn.  Autogenerate cannot be used in a cluster.

This error occurs infrequently.  I'm using a web hosting account.

I modified my web config file with the following code but to no avail:

system.web>
<customErrors mode="Off"/>
<pages validateRequest="false" enableEventValidation="false" viewStateEncryptionMode ="Never" />
</system.web>

Why does this occur and how do I correct it?