Umbraco 7.6.6, running under IIS 8.5

Our content editors notice that often when they create a new article (which has some graphics specified) the system hangs after they try to browse to a different article... or indeed anywhere in the front-end site, too.

F12 dev tools shows that browser is pending the result of this type of call:

http://whatever.com/media/1101/thumbnail_pmc-1x2.jpg?width=500&mode=max&animationprocessmode=first

If I try to view that thumbnail this way, it loads instantly while the first call is still hanging in it's unresponsive tab: http://whatever.com/media/1101/thumbnail_pmc-1x2.jpg

DEBUG level logs just show this:

 2017-10-10 17:02:53,120 [P2816/D6/T57] DEBUG Umbraco.Web.UmbracoModule - Begin request: http://whatever.com/media/1101/thumbnail_pmc-1x2.jpg?width=500&mode=max&animationprocessmode=first.

No errors in the logs, no errors in IIS logs.

Can anybody please advise on how I can troubleshoot further?

Thanks! Tim