As I said earlier I have the same problem, not on FreeNAS, but on a Qnap and I seem to have "fixed" my problem by enabling SMB3 on my NAS (I was at SMB 2.1). It's been 48 hours and I haven't experienced problem connecting to an SMB share (smb://...)
For future reference, here's what I did since the option wasn't available anywhere in the GUI :
www.markuszehnle.com
I will report back if the error comes back, but so far so good.
For future reference, here's what I did since the option wasn't available anywhere in the GUI :
How to enable SMB 3 on your QNAP NAS (TS-659 Pro+) – markuszehnle.com

I will report back if the error comes back, but so far so good.