Hello and welcome! I apologise for the delay of my post.
If you are seeing it at a certain time of day then more than likely the internet service is being saturated by other users that are on the same line/switch as you. Take it from someone that worked in an internet service provider that oversold their service and had to put in new equipment to handle the load
Who is your provider?
Here is what you can do as an end-user; document the following so you can send it to your ISP so they have to do something.
When at all possible, do this direct connected to the modem. What modem do you have?
When it's working fine:
Complete 3 speed tests from 3 different sources:
www.speedtest.net,
www.dslreports.com/speedtest,
www.speedof.me, and
www.testmy.net. Notate the speed for both upload/upstream and download/downstream, as well as ping if it shows. If your provider has a speed test, include that with these 3.
Next, go to Command Prompt, then type
ping google.com -n 100 >
ping1.txt & ping1.txt and press enter.
This will show nothing happening, but what this command does is inputting the data into the text file, then it will display the text file. This should take about a minute.
Next, in Command Prompt, type
tracert google.com > trace1.txt & trace1.txt and press enter. Again, this will not show you the details until it completes the information. This shouldn't take more than 30 seconds.
Once it hits 6PM CST and you start to see the issue, complete the same tasks but with a slight modification... the commands you need to do will be:
ping google.com -n 100 >>
ping1.txt & ping1.txt
tracert google.com >> trace1.txt & trace1.txt
The two >> means that it will add onto the created file so we have one file with both 'good' and 'bad' results per each command.
Hopefully all the information should look different. Please provide the information so I can review it.
Also don't forget to asks friends/family/neighbours to see if they are experiencing the same issue (providing they are on the same internet service and they are somewhat close to you.
Internet providers do have information about the signals to you, to the area, etc as well as usage... it will depend on the company that would actively help you with that information straight away.