Validation of viewstate MAC failed? (v 1.20)

We have been testing nopCommerce 1.20 at GoDaddy for a few weeks and I am now running into some random problems while navigating on either the customer side or Admin side.  The log consistently shows the following:

"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. "

On the customer side, I can reproduce this error during the checkout process after submitting the billing information.  We are using the USPS shipping method with the correct settings, so I'm hoping this is not the issue.

On the Admin side, it randomly occurs every once in a while when trying to update something.  Yesterday it was when I was configuring a Category.  Today it was when I was trying to clear the Error log!  Go figure.  I've recycled the Web Application a few times but it doesn't appear to alleviate the reoccurance of this problem.

I'm not a .NET developer and really don't know what to look for if it is a coding problem or a site configuration issue.  Since we are new to GoDaddy, my first inclination is to blame the environment under which our site is running.  Since there is a lack of VIEWSTATE issues on the forums here, I assume that this issue isn't wide spread.

This site is hosted at GoDaddy using the Windows Shared Hosting running .NET 2.0 and 3.5 with IIS 7.  Any help or pointers would be greatly appreciated.  I'm hoping GoDaddy support can help on this, but it's GoDaddy and NOT their eCommerce solution ;)

Additional Details that might help:

Exception of type 'System.Web.HttpUnhandledException' was thrown.  
Exception:  System.Web.HttpUnhandledException: Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.Net.WebException: The remote server returned an error: (405) Method Not Allowed. at System.Net.HttpWebRequest.GetResponse() at NopSolutions.NopCommerce.Shipping.Methods.USPS.USPSComputationMethod.DoRequest(String URL, String RequestString) in D:\Work\Own\NopCommerce\Solution\Solution\Shipping\Nop.Shipping.USPS\USPSComputationMethod.cs:line 154 at NopSolutions.NopCommerce.Shipping.Methods.USPS.USPSComputationMethod.GetShippingOptions(ShipmentPackage ShipmentPackage, String& Error) in D:\Work\Own\NopCommerce\Solution\Solution\Shipping\Nop.Shipping.USPS\USPSComputationMethod.cs:line 63 at NopSolutions.NopCommerce.Common.Shipping.ShippingManager.GetShippingOptions(ShoppingCart Cart, Customer customer, Address ShippingAddress, String& Error) in D:\Work\Own\NopCommerce\Solution\Solution\Libraries\Nop.Common\Shipping\ShippingManager.cs:line 259 at NopSolutions.NopCommerce.Web.Modules.CheckoutShippingMethodControl.Page_Load(Object sender, EventArgs e) in D:\Work\Own\NopCommerce\Solution\Solution\NopCommerceStore\Modules\CheckoutShippingMethod.ascx.cs:line 123 at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) at System.Web.UI.Control.OnLoad(EventArgs e) at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace --- at System.Web.UI.Page.HandleError(Exception e) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest() at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) at System.Web.UI.Page.ProcessRequest(HttpContext context) at ASP.checkoutshippingmethod_aspx.ProcessRequest(HttpContext context) in c:\windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\store\8623c638\32df928f\App_Web_frgz0m7k.34.cs:line 0 at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

It looks like part of the problem is caused when trying to check out using USPS.  Apparently v1.20's version of USPS isn't compatible with the latest version of the USPS Shipping API's (3).  And once this viewstate issue pops up, I think it impacts .NET performance in some way on the GoDaddy servers to cause other random occurance.  I've tested it now after a few days of not playing with the store and was only able to reproduce the Shipping issue. No other strange behavior has occured.

So hopefully I can update the v1.20 USPS shipping method file with the v1.3 file and be on my way!  If not, I'll have to try updating the entire software package to v1.3.  Kinda flying blind here (excpet for reading the install procedures), but I look forward to learning more about nopCommerce through this process.

Ever have any luck? Had the same problem and looking for answers. I'm on version 1.50.