Readynas 104 drops out when copying to it

I have a readynas 104 configured with four 4tb drives in raid 5. I've managed to copy about 6tb onto the device without issue but now whenever I go to copy something onto the box, lets say a single 4gb file, it starts out ok and will copy around 1gb of the file at 31MB/second but then the connection drops out completely and I get an error message saying:

There is a problem accessing Z:\....
Make sure you are connected to the network and try again.

Now I've obviously mapped drive Z: to the box and when I say it drops out, I can still ping it's IP address. I have tried both NIC's with different cables and in different ports on the router with exactly the same result every time. I am running two other buffalo NAS boxes off the same router and they are working fine.

I can view the files etc over readycloud and check it's performance without problem and in reasonable time, but then as soon as I copy a file over in explorer, readycloud all but freezes and wont let you navigate through the various screens. And once I try to copy a file across I can still view files and directories on the box in explorer, albeit it takes about 30 seconds or so to load each directory which is ridiculous, but before attempting to copy each directory will load almost instantly as you would expect.

Any ideas?