If I leave it off for several days and turn it on, it behaves normally. Very strange stuff.
The above quote leads me to think hardware issue, not software. Without knowing more, I'd guess a failing power supply...
Try posting your system specs including FreeNAS version, FreeNAS configuration and of course
a complete list of the hardware used.
Here's mine for an example;
Case - Mercury S8, CaseLabs
PSU - AURUM S 400 Gold, Fortran (FSP Group)
Motherboard - X11SSM-F, Supermicro
CPU - Pentium G4400, Intel
Memory - 16GB (16GB x1) M391A2K43BB1-CPB, Samsung
Boot Device - X25-E SSD 32GB SLC, Intel
Hard Drives - Six 3TB WD30EFRX, Western Digital
NIC - X520-DA1 10Gb, Intel
Vol. Config - Single vDev RAIDz2
FreeNAS-9.10-STABLE-201605240427 (64fcd8e)