I have the same problem with DSTM 0.5.6, 0.5.7 and 0.5.8. Stable and fast as hell, but after one hour it starts to make less and less sol/s.. in three hours I'm getting half rate (I start with 680-700 sol/s and after 4 hours I end with 230 sol/s)... so my only solution is restart the miner. All the time no more than 60ºC, fan speed at 60%. I have two rigs, same specs and the same problem: Asrock H110 PRO + 13 Nvidia 1080Ti (up five different brands, mixed). Every rig have 4 PSUs: 3 x 1000W + 1 x 1600W and just one cable to power every riser (4 pin molex version) so no problems about power or temperature. I use simplemining without overclock, just power at 200W, no extra mhz in gpu core or memory. Is it a configuration problem?