I have a TD-W8980 myself and I use some experimental patches, which have been tested thoroughly and they improved the wifi a lot and also the overall cpu performance and ethernet speed. I use it with a external hdd for torrents and samba mainly because Internet is routed from HH5A. Btw you can also try these patches and hopefully they can improve the performance of your router in general.
At first it was thought that your flash may be corrupted so for that I would choose the overlay option with slightly changing the overlay part. At boot router will need to mount the USB and then, unless restarted, everything runs from the USB. I just modified the copy command to include rom and overlay.
mkdir /mnt/upper
tar -C /rom -cvf - . | tar -C /mnt/upper -xf -
tar -C /overlay -cvf - . | tar -C /mnt -xf -
You just need to execute the above commands in order and then everything should be in USB and your router should work fine.
Have a look here