just updated my firmware to same TD-W9980_V1_160125 it's rubbish. I've got 4294966676 errors on the upstream, downstream keeps downgrading it self to the point I was at 284Kbps. But I can't downgrade...nightmare!

I regularly have issues in YouTube videos while trying to skip parts of the video - often forcing me to fully refresh the page.

That doesn't sound good - any packet loss on a BQM?

Your actual number of U/S errors is 620:

2^32 = 4,294,967,296 - 4,294,966,676 = 620

As explained above the error count is being misrepresented as it starts at the negative max 32bit integer and counts down to zero. I've seen this issue on other modem/routers.