I had random reboots on some of my rigs after updating to CZM 9.0/9.1. Try to underclock and undervolt your cards a little (for example I use my 470 4GB with 1200Mhz GPU clock + 1000mV, 1750Mhz memory clock + 950mV, and -5 for voltage offset in WattTool). That solved me the stability issue.