Sync stutter with dual GPU?

Having an issue with a dual NVIDIA GPU set-up. More specifically, I have 2 x K6000 to output an 8K blend. At 7680x2160 (using UHD) monitors, it runs fine. But at 8192x2160 (using 2 projectors) there is what appears to be a sync stutter (every second the performance drops from around 60fps to 6-7fps). The only thing I can think of are sync settings (driver, card)…

One note: the only difference between the two displays is that the UHD monitors are synced at 59.98Hz and the projectors at 59.94hz… But either using vsync or overriding the sync within the nivida control panel should do the work, no?

I don’t think the resolution difference is responsible for the issue (as it happens I am trying pretty simple test patches).

Any ideas of what to look for?

For the record, it seems that the NVIDIA sync override worked after properly applied. Driver versions matter, of course, and I’m told that the current most stable version for mosaic handling is 333.11.