ViewState corrupted?

My site is randomly firing off a "ExceptionHttpException : Unable to
validate data. " error on some of its forms.
-------
HttpException : Unable to validate data.
http://www.developerfusion.co.uk/Members/Private/EditProfile .aspx (Referred
by http://www.developerfusion.co.uk/Members/Private/EditProfile .aspx)
at System.Web.Configuration.MachineKeySection.GetDecodedData(By te[] buf,
Byte[] modifier, Int32 start, Int32 length, Int32& dataLength)
at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString)
-----------
I am also able to reproduce this error at one of our stores machines,
meaning all of the computers at that location return this error for one
particular application. The other application seem to randomly return this
error. The site is not running on a web farm and I've tried the standard
solutions of specifying a fixed machine validation/encryption key, and I've
also tried disabling the view state mac entirely, but the problem persists.

This application does not have any Server.Transfers in it, so I've ruled
that one out. All post's are done within the same page and then upon
success response.redirect is used to send the user to the next page.

Does anyone have any ideas what setting might be triggering this? Could it
be that my viewstate is corrupted for this application, and if so, how do I
fix it?

Thanks!
Velvet