Asp.net4, C# with MSSQL based website - *Issue to solve*

We have a website that has been live for around 2 years with various bits of work carried out over this time. However we seem to have an issue or 2 that needs resolving. Our users login to our website to make changes to their adverts, when doing this Sometimes, when they click the save button they get the 404 page. Or it takes them back to the login page. Its the same if a new user places an advert, they type out their advert (this could take a minute or 2) and when the click to proceed to payment they either get taken back to the home page or get the 404 page. These instances above do not happen every time. We as admin have tested certain parts of the site and we sometime get the 404 page too (when logged in as admin) and sometimes we are logged out. We can and will show you how our website works via a platform like Teamviewer. Please only bid on this project if you wish to fix the issue. NO FIX NO FEE! So please ask questions and we will be available to help. Thank you.