Form authentication random timeout

Hello,
Im having random timeout in my Webapp.
Im running the framework 3.5.
Im using nested Usercontrol in Updatepanel.
Im setting my authentication cookie at 60 min and change the value in my Web.config as in this example: http://msdn.microsoft.com/en-us/library/aa480476.aspx

 
Ive added the machine key in the Web.config as in this example:
http://msdn.microsoft.com/en-us/library/ms998288.aspx#paght000007_machinekeyexplained
Ive also tried to use a static machine key generated by a third party.
For the life of me, i cant figure out why it randomly timeout.
It can be allright for a few session and then BAM, get redirected to the login page, and there is no exception raise.
 
Thanks for any input!
Mat

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