I was active on this thread a while back. Thought I'd return to have a look.
If you have set a 100Hz overclock on the monitor, then 60fps and below you will see the scanlines.
If you have set a 85Hz overclock on the monitor, then 50fps and below you will see the scanlines.
This is assuming that G-sync is ON in the NVCP.
This explains why when you add a second card, the scanlines go away.