my latency on my cable connection fluctuates between 50 and 400 over hours or bounces up and down every few seconds

i get 768 download and 128 upload

how can i improve or fix this?

My first question to you is what is your speeds SUPPOSED to be, that sounds VERY low for cable internet. I can't give you any suggestions until i know your proposed speed, because, if your speeds supposed to be 768 down by 128 up, then your right on the money, if it's supposed to be a 4Mb/s line, i'd be calling a tech support supervisor and considering the fact there may be engineering issues.

Get back with that info, and read that link other person left, then if u have  m0ore questions feel free to post again, but at least give us your proposed line speed, and your network/router description, (dont risk your security).

yeah so your getting packet loss as more users of your network node sign on (peek hours).
other then replacing network devices, there isn't much to fix that.