Make sure that your motor pulley isn't slipping. I had a similar problem a while back with my Aries.
VPI SDS speed issue
I am having trouble with a consistent speed on my Aries 2 two pulley setup. I find I have to at times set the frequency to almost 66 to get it spot on. At other times it may be at 63. I read somewhere about the voltage preset may need adjusting. How is that done or is there something else I should be looking to adjust? The motor stas at 115, shifts to about 84after warm And then maysettle down to about 72 Hz. Any help?