Happy to report that this is issue is resolved!
While investigating, I logged in as a different user (the guest account) and ran FSX - and I was pleasantly surprised - no scenery issues, everything smooth - which was a breakthrough moment.
When I logged back in to my regular account, the issue was back.
So I went digging in the user profiles and noted in C:\Users\Jonathan\AppData\Local\NVIDIA there was a directory called "GFExperience.exe_Url_tgm3mttmt5itflhj0kykoi5xkjy53x4g" that did not exist for the Guest user. In the directory was a directory "11.10.13" and within that an xml config file called "user.config". This had a recent create date, coinciding with the time I noticed the issue. Not sure what event had created this file.
I renamed the "GFExperience.exe_Url_tgm3mttmt5itflhj0kykoi5xkjy53x4g" directory and restarted FSX - and the issue was gone. Hope this helps someone who may have this issue.
cheers
Jonathan