Is this happening when you are doing a parity check? If so, when such high activity task kicks in, the PSU is undergoing an immense load since ALL of the drives are spun up and are performing a set of validation tasks.
As such, I'd recommend calculating, approximately, how much power do you consume at least on drives and looking at the power supply thereafter.
For example, I have 16 x 1TB Seagate Drives:
Power consumption per drive in my case:
1.8A x 12V = 21.6W
Total HDDs: 21.6W x 16 ~= 350W
So if you have a decent CPU and maybe a video card in the mix, then you'll need a powerful PSU to handle that. Remember, the PSUs can only sustain 80% efficiency for a prolonged duration of time.