On a 3-monitor nVidia GTX 680 set-up, the RH monitor has, until this 7.1 update, been used at the preview monitor - selected from within PP preferences.
After the 7.1 upgrade, PP displays intermittent behaviour.
From the right, my three monitors are indentified as 1,2,3 (the RH monitor being 1) . When 1 is selected as preview monitor in PP, monitor 3 displays the preview. No other option places the preview on the right-hand monitor.