Slow internet-kicked off network

I have had preferred service for over 6 years and never had an issue with the internet until about a month ago.  Our main computer, along with our mobile devices are continuously getting kicked off of our wi-fi network and being unable to join.  When they are connected, it is incredibly slow.  I have included a screenshot of our past speed checks this week.  Today's was pitiful.  I have tried resetting the modem (Motorola SB5120) and airport device connected to the internet to no avail.  Any help would be appreciated.