I must admit that I'm stumped as to why the ALT+TAB is not always on top. As a test, create a new administrator account to see if it works properly in it.
If it does, then you can copy the contents of all folders (including hidden
AppData folder) from your old account's C:\Users\(user-name) user folders into the corresponding new account's C:\Users\(user-name) user folders to copy over most of the old account's settings. After everything checks out ok, you can delete the old account from within the new account.