When the guest use NAT networking all CIFS connections in NAT mode come from the same IP, we suspect that NetApp starts disconnecting certain connections once newer connections come in.  According to the following article, this is a known issue with NetApp devices and CIFS connection from behind NAT: http://www.gossamer-threads.com/lists/netapp/toasters/11009

A setting on the NetApp that does dupe detection on source IP addresses can be modified. It immediately improves the situation of lost network shares. 

It refers to a KB article (NetApp login required) : https://kb.netapp.com/support/s/search/All/1011549?language=en_US

We implemented this for a customer who set the following option on their filer options: 

cifs.client.dup-detection off

This solved the issue of lost CIFS shares.