VM FUSION 5 - System Slows Down to a Crawl

Hi-

I have VM Fusion installed on a newer iMac. I have two machines - a Windows 7 machine and a Windows 8 machine. I do not run these machines in parallel, and they worked fine separately for a while. But now all of a sudden they both have become so bogged down they are unusable. Is there a reason for this? Do I need to erase and start over? this is very frustrating, my iMac is operating fine