I am paying for 300 speed and getting less than 100

I am paying for 300 speed and only get less than 100 upload and less than 10 download. I am on ethernet.  I have internet only plan.  Direct to modem.  I had a tech come out and he said all is well with everything.  I got a new modem and cable...

I got over 200 speed for a couple weeks, now back to less than 100 

I have corporate clients and my internet isn't stable.  Is Cox reducing speeds?  Is Cox reducing speeds on weekends?  Any suggestions or do I need to find a new provider?