So I have been scouring the logs and haven't found anything useful. The windows event log contained some warnings related to the USB drivers but this hasn't helped much my VM has no USB devices connected and I have tested with and without a USB controller and there is no difference. As for the host logs I have been searching through them too but nothing is jumping out at me.
However I have created a new Win7 VM with Paravirtual scsi controller and the VMXNET 3 network adapter (still running V5 update 2) and this is looking good. Its been running for half a day so far without skipping a beat.
While not ideal as I will have to rebuild a few VM's and I would prefer to know and understand the route cause of the issue this looks to be a solution but I will be monitoring for a few more days before I declare this as "fixed".
Thanks for all your help!