Hey guys.
I have been suffering from the same issue and I had been changing the settings around daily just to see if any thing helps. I just want to dump my findings here.
So if iGPU is disabled, the glitches almost starts immediately when the system is booted (Like triggering previews or opening the Photos app.)
With iGPU enabled, it kind of delayed the glitch for some random amount of time. Web browsing doesn't trigger the glitch, but opening Photos does.
With the determination of bricking the card, I tried flashing some random GTX660 vBIOS to my card and see if anything sticks. Naturally it didn't work, and yes I bricked my card. Then I realise how easy it is to bring it back so I flashed it again it came back to life.
When I was cleaning my Clover folder I realise something called CSMVideoDxe.efi, as the word video suggested it is kind of a video driver for system doesn't support UEFI GOP in Clover, whatever that means. So I turned off CSM in BIOS. Boom! It doesn't boot, BIOS asking me to turn back off as the GTX660 doesn't support UEFI GOP. With some random googling I realise you can actually flash the UEFI GOP module to the card to add this feature, so with that determination again, I flashed the card.
I got video output. Go to BIOS and turn off CSM. BIOS doesn't complain this time. Boot to High Sierra, delete the CSMVideoDxe.efi from the EFI. For some reason I REALLY don't get, I got noticeably fewer glitches. The photo app doesn't crap itself like befor
When I say few, glitches still happens, like doing file previews sometimes. iMovie is almost 100% chance of triggering glitches. But I don't use iMovie every day so I'm happy for the moment, at least illustrator is more usable
I don't have to iCloud Drive the goddamn file to my 2012 Non-Retina MacBook Pro just to see the goddamn file.
I'm not responsible for any GTX660 bricks, but this is a old card now so what the heck.