Over 60° the cards start to throttle. Only 1-5 %. At 70° they throttle even more. Keep below 60° to get Full Power. Thats my experience with 3 setups with 1060 3/6GB, 1070TI and 1080TI.
And thats why your hashrate goes down after a while, cause VRMs/GPU and MEM is hot and the card slowy begin to throttle.

My setup after hours of mining:

ID   DEVICE NAME          °C   ∅ Sol/s   ∅ Sol/W   ∅ Watt
0   GeForce GTX 1080 Ti    50   652.34   4.11   158.67
1   GeForce GTX 1060 3G   50   277.33   3.86   71.92   
2   GeForce GTX 1070 Ti    52   491.61   4.58   107.23
3   GeForce GTX 1060 3G   49   274.56   3.83   71.74   
4   GeForce GTX 1060 3G   54   274.18   3.82   71.71   
5   GeForce GTX 1070 Ti    46   492.15   4.58   107.49
6   GeForce GTX 1060 3G   54   277.34   3.87   71.68   
Total   -                              2739.51   4.09   660.44   

The ambient temperature is 12° (thx winter)

Im using Win7 and the cards have these settings:

GTX 1060 3GB hynix: GPU +240 MEM +400 PL 60%
GTX 1070 TI : GPU +180 MEM +600 PL 60%
GTX 1080 TI : GPU +160 MEM -100 PL 60%

Theres a little room for stability. 1080 Ti runs since 1 year 24/7 with these settings without hassle. I prefer a rock stable system.
In a downtime you are losing money. So a little lower performance is okay for me.