Mining of virtual servers will always be poor and susceptible to changes in HR. With virtual servers you are sharing CPU resources, and with CrytoNight, you need 2MB CPU cache per thread for optimal performance. A virtual server will never give you exclusivity over CPU cache, hence you'll see changes. Further, most hosting companies forbid mining, so If they detect it, they'll drop your resources.

TL;DR never mine on virtual servers.

Mining of cloud based virtual servers will always be poor and susceptible to changes in HR. With virtual servers you are sharing CPU resources, and with CrytoNight, you need 2MB CPU cache per thread for optimal performance. A virtual server will never give you exclusivity over CPU cache, hence you'll see changes. Further, most hosting companies forbid mining, so If they detect it, they'll drop your resources.

TL;DR never mine on cloud virtual servers.