Default Nexus 7 bootloop

My friend gave me his Nexus 7 to fix as it was bootlooping. I have been able to wipe it and reflash the boot loader. However, when I try to flash the stock image (jwr66y or kot49h) it gets stuck at 'sending 'system''. I have tried waiting for it but it just always sticks at this point.

I think it is the fast boot not working as it keeps on saying: Failed (command write failed (Unknown error)). So I have to unplug it and put it back in to another port to work. However, even with this error, fastboot devices returns the device number.

Is there a way to fix this problem or is it a fault with the netbook, tablet or cable?

Thanks!

Sent from my Nexus 7 using xda app-developers app