Security :: Postbacks With Authentication Causing Errors?

After adding personalization and authentication to my site, I was getting pitched out of my application( which is on a hosted server), and sent back to the log in screen . I disabled authentication to see if I could figure out what was going on. I found that

I was getting a system 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.

This error has been reported elsewhere [URL] so I made the following change to my web.config

<pages theme="Theme1" enableViewStateMac="false" enableEventValidation="false" viewStateEncryptionMode="Never">

This seems to have worked to a point, because with authentication DISABLED, I no longer get the error. But when I RE_ENABLE authentication, I get pitched out randomly back to the login screen. Time out is not an issue. It only happens when I move very quickly through screens that are doing a lot of postbacks.

This is the authentication stuff from my web.config

<authentication mode="Forms">
<forms name=".ASPXAUTH"
loginUrl="~/Login/Login.aspx"
protection="All"
timeout="3000"
path="/"
requireSSL="false"
slidingExpiration="true"
cookieless="UseDeviceProfile" />
</authentication>
<authorization>
<deny users="?"/>
</authorization>

I tried AutoDetect on cookieless as well. No change. I would appreciate ideas on how to troubleshoot this problem.