We are facing strange issues with my custom web part. This web part is embedded on the page and whenever user tries to access this web page first time in a day he gets "403 Forbidden error" then he clicks browser back button and web page loads perfectly with web part. Afterwards whole day this problem never comes. It happens same way again from next morning.
User has full control on the site so I don't see any such permission issue. This web part reads documents from multiple lists and display on data grid. User has full control permission on all the lists so it's also fine. I tried to comment the web part code and still he gets forbidden error when he logins first time.
I don't know what else I should look for .. Any help will be much appreciated.
Note: It's not just dev machine but it's happening on prod server as well. Also, It's not for one but happening for many users.