I'm having a similar problem. I have noticed that my windows and macbook keep disconnecting from the server. If I go investigating at server it reconnects.
Mobo: Gigabyte EP45-UD3P
CPU: Intel(R) Core(TM)2 Duo CPU E8400 @ 3.00GHz
Mem: 8 Gb non-ECC
Power supply: 500+
Drives: 2 120+ gb Sata drives (BOOT)
1 4 TB drive Seagate (from a Seagate NAS that the controller fried on.)(Data Drive)
I've only started having this problem since 11 went to release version. I'm using this at home and at the moment I'm the only one using the FreeNAS system for storage, so the hardware may be old but FreeNAS hasn't given me any problems till of late. Last week I had to recover data from the drive that said it was last accessed from another system, because the FreeNAS went into a Panic mode and I reloaded the software before I knew what had caused the issue, or that my drive may have been the cause of the panic. I even had a panic mode start today, but luckily I had my data on another drive as well as the FreeNAS system still so I wasn't panicing :p I am in the process of copying data to the FreeNAS system now and it hiccuped about a 1/4 of the way, disconnecting my Windows 10 machine from the share.
I have set S.M.A.R.T to do a long scan to see if the drive is failing. Hopefully some info will come up as to why I get a hiccup from the server.