If your RAM is giving you errors you can try this, it normally works...
Could you post the specific RAM make and model number you have?
1) Make sure your RAM is not being overclocked. Slow it down in the BIOS. So if it's running at 1600, turn it down to 1333 or whatever is the next lowest setting, it doesn't matter at this point what your RAM speed was rated for, you are only trying to figure out if you can get it to pass MemTest at any setting.
2) Now run MemTest on it. You want to run MemTest for 3 complete passes if you can and that can take over 24 hours depending on your system. At a minimum run it one complete test cycle.
3) Now let's say you are able to get steps 1 and 2 to work then you might be thinking that the fast RAM you bought was a rip off. Sometimes that is true but most of the time it's a voltage setting issue. I don't recommend you make these changes for a FreeNAS box unless you know what you're doing because you can fry something very quickly. Go searching the internet for "RAM Overclocking" or something similar.
If you can remedy this problem I would first make sure your motherboard was compatible with the RAM.
If you feel the RAM should be compatible, most RAM have a lifetime warrantee so you should try to locate which stick is bad and try to get it replaced.
Sometimes it's the motherboard so you might have to get a new one.
Last thing, are you sure the power supply has the capacity for your system? A cheap power supply can cause all kinds of problems.
I know I'm asking a lot of things but you didn't give much information to go on so I'm trying to cover the bases.
Good luck.