Thank you for your quick and helpful response - I am most grateful. I had updated the Nvidia driver (first update for months) and it does seem to be the new driver that was the problem (Don't know why I didn't think of that!) Here's what I found:
With 436.48 installed - disabling SLI removed the flickering, enabling SLI restored flickering.
Rolled back from 436.48 to 416.94, with SLI enabled: no flickering.
Updated 416.94 to 425.31, with SLI enabled: no flickering.
I'll stay with 4
Nvidia 425.31 doesn't have the driver bug with SLI
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