I have been mining ZEN for the last 2 months with my 6x1080ti asus strix (asrock btc h81, 2xEvga 1000w g3), and 10 days ago it started crashing all the time, even the whole machine started restarting. I checked cables and changed all risers and still the sam. Then I took one card I suspected might be the cause of problems and put it on my desktop machine so I could check it solo. It worked perfectly for 3 days and the rest of the rig with 5 cards worked perfectly so I thought it might be that one of the PSU is having problems with supplying 3 cards. I cleaned all the components with air compressor and put the 6th card back and now everything is working perfectly for the last 5 days...
So...we should regularly clean our 1080ti's :-)