Uploads make internet cut out and upload speeds extremely slow

I work from home and a big part of my job is uploading audio/video files. We have very low upload speeds, and the internet disconnects every few seconds every time I try to upload files (and then I have to start the upload again - our work system doesn't allow it to just continue uploading after a blip). I am unable to work with this happening.

We had a tech come out today, and he and his supervisor saw the slow upload speeds, but didn't seem to have any idea of what was happening. They replaced our equipment, gave us a new splitter, checked everything again, and it's still happening. Supervisor said he was going to issue a ticket, but I need this fixed quickly. What else can we try? Or how can I otherwise escalate this issue? If it is not fixed within a week, I'll need to cancel Xfinity and request a full credit for my lost work time/first bill. 

This does not happen on other internet/wifi that we've had, so it has to be Xfinity, not me/my laptop that has the issue. It's extremely frustrating that no one has been able to give us an answer.