VDI is automatically going to down and become 'Saved' state

We are testing the HYper-V 2012 and Windows 8 Enterprise 32 bit. We have created Personal Desktop collection and Created VDI. But this VDI's are frequently going Down automatically and if we check in Hyper-V, the VDI state is showing as 'Saved'. Anybody can help us as we are going to decide whether to go with Microsoft or VMware for Virtualization

Thanks in advance