I can't offer any useful advice, but what you describe is something I've not seen in Gnome, ever, and I have used it since 2.4. Admittedly I am on 6.3 instead of 7.x, but that should not matter much.
Gnome for me stays up for a couple of months at a time; the crash then seems to be related to X11. I'd look there, perhaps looking at the video driver.
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