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.