1-5 mbs download speed

For a couple weeks we've been having sporadic noticeable  periods of slowness. My speeds were almost always 30mbs down, 20mbs up. That's slow considering I pay for 300mb down. Now it's basically unusable. This is critical because I work remotely and I can't do anything that needs to access the Internet, which is a big deal because I'm a web developer and use the internet throughout my workday. I had a tech out who said the problem was in the wiring, but it's weird that these problems started when an Xfinity contractor did work on the building equipment. I'm having no luck with the automated troubleshooting. I hope that there's someone here who can point me in the right direction to get someone to resolve this issue.