HTTP error 404 on wp-login.php

My first forage into WordPress although I’ve been supporting CMS systems for years on Windows. New installation running WP on Windows 2012 Server with IIS 8. Works a treat except for one annoyance. When one returns to the website after a time, you get a 404 error if you try to load http://www.mywebsite.com/wp-login.php. If you refresh the page a few times, the logon screen does finally appear.

There are lots of posts on this same error but most of them appear to be temporary fixes and I’m looking for something more permanent. I’ve got another pilot site on the same server and it has the same problem.

I know how websites and IIS work reasonably well internally as I’m also a web app developer and I’m guessing that it’s something to do with the worker process ending after a period of being idle. There are lots of talk of permalinks but I’m not sure this is the problem with IIS as http://www.mywebsite.com/wp-login.php isn’t a redirection at all – wp-login.php exists at the root of the website. Using /admin would be a redirection I assume but I’m not using that URL.

Whilst I’m very familiar with IIS & web apps, that’s more in the ASP.NET world and my PHP knowledge is sketchy.

Looking first of all for why WordPress or PHP might be throwing up the 404 error at all?