keep asking login window and show 401 unauthorized message in browser ,when try to login with a admin user or spfarm user

In sharepoint 2010   farm

we have a web application on port 80 it has 

claims based authentication and annonymous access enabled and we created a sitecollection in same web application

but it has not enable anonymous access,

here we have two problems when we login to web application

1) when we log in to default site collection with a authenticated user (ex. sitecollection owners are reader permissions)

it  allow login but some times we refresh the page  again it asks authentication window and if try to login with another user

it not allow to login to site and show 401 unauthorized message in browser

2) and also in other site collection some times we login with spfarm or sitecollection admin user it keep asks login window

and not allow login, even we sign in as diffrent user, when we signout it closes the browser and when open browser and site again it login autmatically 

when we login to this site collection 

when we browse sharepoint pages , and some times refresh the pages 

adil