Interlacing output "drift" every ~15 secs. nvidia-settings -q RefreshRate still reports 60.05, regardless of what the actual refresh is as reported in Xorg.0.log.
Some fuzzy math, maybe someone more technical can break it down. Actual TV refresh rate from TV and Xorg(and let's assume this is what the hardware is pushing): 60.10 Hz. Nvidia software driver hardcoded refreshrate at 60.05.
60.10 - 60.05 = .05 difference
60secs / .05 difference = 12 secs
I may be way off base, but this makes sense to me.