OSX Mountain Lion VM running very slow

I have two VMs - Windows XP and OSX Mountain Lion. The Windows XP works fine. The Mountain Lion simply hogs CPU - reaching 170% - and takes ages to load and operate. I have seen threads which suggest uninstalling Paralells and reloading it. I have done this but it hasn't improved. I need a fully functioning Mountain Lion VM to run older software before I upgrade my iMac to Yosemite. Can anyone suggest what might be wrong please?