Intermittent Invalid Viewstate Error in ASP.NET Web pages

Error:
The issue presents itself with and error similar to this:
c:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\mdcr\5c7639a0\5981344d\App_Web_oja12imh.0.cs Line: 0 

Stack Trace: 


[FormatException: Invalid character in a Base-64 string.]
System.Convert.FromBase64String(String s) +0
System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +77
System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String serializedState) +4
System.Web.UI.Util.DeserializeWithAssert(IStateFormatter formatter, String serializedState) +37
System.Web.UI.HiddenFieldPageStatePersister.Load() +113




[ViewStateException: Invalid viewstate. 
Client IP: known ip
Port: Port number XX
User-Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.04506.648; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729; InfoPath.3; .NET4.0C; .NET4.0E)
ViewState: 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...]


Cause : 
There are many reasons for this error :

1. Web Farm or server cluster.
2. Form Posts
3. Application pool recycling 
4. Proxy servers and virus scanners

Resolution :

1. Use specific machine key in your web config. we can use these tools Link 1 or Link 2. Sample Key Node:
   
   <machineKey validationKey=”56AB7132992003EE87F74AE4D9675D65EED8018D3528C0B8874905B51940DEAF6B85F1D922D19AB8F69781B2326A2F978A064708822FD8C54ED74CADF8592E17″
   decryptionKey=”A69D80B92A16DFE1698DFE86D4CED630FA56D7C1661C8D05744449889B88E8DC”
   validation=”SHA1″ decryption=”AES”
   />
2. Post to the same aspx page.
3. Turn off the firewall and antivirus software.
4. Adjust the settings on the application pools so that recycling is less likely to occur at peak periods.