Network Connection Drops when Transferring Large Files

In short, when I try copying large files ( > 1.0 GB) from my PC (wireless AC) to my NAS (ethernet), the connection drops sometimes (the sometimes is the irritating part.
- I have a Asus wireless AC adapter on this PC.
- It is more likely to happen if I am downloading other files (from the internet) while I am trying to copy this file (and this makes sense), however I would think it would just go slower (when I have more network traffic) - not just drop the connection.
I tried...
- Replaced every cable to Cat-6 within the past month.
- On my adapter I disabled the IPv6 (as I read that can cause connectivity problems).
- Updated the router's firmware. The router is an Asus RT-AC68u

I have a second PC with a wireless AC adapter and tried copying a large file to my NAS (ethernet), and the connection also dropped.
I don't know if this is a Windows 7 issue or if my router settings need to be adjusted to correct this. I did disable the IPv6, and I thought that would take care of it. If anyone can offer some suggestions, that would be appreciated.
Thank you.

Have you tried LAN to LAN transfer (don't use your Wireless NIC) and see if the drops happen? If not, there might be issues with WLAN and you may need to update drivers for your Wireless NIC.

I did try a transfer between 2 "wired" devices, and I was able to get the disconnect issue to happen.

One note - I had to do a few file transfers at once for it to happen.

It is like the connection gets saturated with data and disconnects (Although I'm not sure if that is what is supposed to happen).

Maybe router's NVRAM is saturated and needs to be cleared. Clean NVRAM by setting the router to factory defaults and then manually configure to your current configure. Don't load a configuration file since the issue might reappear. If the issue still happens after factory reset and manual configure, it maybe getting hot under heavy load which results to disconnection.