Drive drops out when doing timemachine

I have a ReadyNASRNDP400U which has 4 x ST2000DL003-9VT166 in RAID-X. Firmware Version: 4.2.28 

I opened a ticket when my unit was under warranty and I don't believe I managed to get any help or info relating to my issue.

Whenever I try and do a TimeMachine backup to the NAS over gig ethernet where the device needs to run "high" bandwidth transfers from my SSD based Macbook Pro for some reason a drive, normally drive "3" drops out and I lose my redundancy. I have the device set to shutdown on drive error, so when I come back to check the unit is normally off and I have to pull the drive, reboot it and slide the drive back in, and it does a raid rebuilt. This seems to happen "every time" and I can't get a complete backup.

This doesn't seem to happen when the transfer is slower ie. if I try and do it over WIFI it seems to work/complete but takes forever. I have been able to reproduce the issue every time I attempt this, and it's been happening since I can remember. If the timemachine backup transfer goes >100mbits for any length of time, it seems to happen.

If I try and transfer a quantity of files at similar transfer speeds the unit seems to function properly, it seems like the issue relates to "fast" timemachine backups only.

I don't seem to be able to see anything when ssh'ing and poking around, so I thought I'd ask. I don't see any reason for it to be doing what it's doing, and again it only seems to happen when doing a TimeMachine backup.

Yes, the device is aged, yes the drives are aged, but the only time I can ever remember having to do a raid rebuild is when trying to do the backup as above.

Any suggestions or anything I can try would be most welcome.

Thanks