That's strange: JFK has been out since about 2 years, but nobody else reported this before.
From your description, it seems like video memory corruption, which might also be caused by overclocking or overheating (not unusual in August...), also because you said it happens after a while. Do you have a way to monitor the temperature inside your PC and/or video card ?
Also, it might be a video driver problem: are you using the latest drivers ? Also, if you are using the current version of the Addon Manager, try to enable the following setting:
HIGHMEMFIX=1
Then press the "Save to FSX" button to save the setting, restart FSX and see if it makes any difference.