I've got a GTX-580 powering a newer 1920x1080 display and a fairly old LG at 1280x1024.

Using any driver lower than 358 both displays work fine. All the drivers after 358 cause the LG display to exhibit refresh rate flickering at its native resolution and refresh rate (60.02Hz). Any resolutions lower than this work as they should at all their supported refresh rates.

I've tried using both HDMI and DVI with the same result. parse-edid says the EDID is only partially read, but that's also the case for the working drivers; the monitor shows up as it should in the Nvidia settings and xrandr.

It's definitely a driver issue, I've tried various display managers with the same result. I've had no success changing any of the settings in the Nvidia manager.

I tried adding custom modes in xrandr to see if setting the framerate to 59 or a flat 60 has any effect, but (possibly due to the EDID), I just get this error:

X Error of failed request: BadMatch (invalid parameter attributes)

Same behaviour in both 16.04 & 16.10, although 16.10 won't work with drivers < 367 for me.

Any suggestions to overcome this?

Proprietary drivers can only be fixed by, in this case, Nvidia. Until they provide a fix, your best option is to use one of their older drivers that weren't giving you this problem

I knew that'd be the case for a proper fix, and I'm honestly not expecting one from Nvidia unless it's accidental - I was hoping someone might have a workaround.

Having looked into this further it's not refresh rate flicker, you apparently don't get that on flat-panels (I'm an old fart, I spent too long with CRTs) so I'm guessing it's backlight flicker instead. There seem to be quite a few people experiencing it in 16.x, but every case I've found so far has involved an i915, not an Nvidia.

Looks like I'll stick with the Nouveau drivers, even though they're far from smooth. Cheers for the reply.