|You're putting far too much faith in those speed tests. They're not all that accurate to begin with and are always affected by congestion. The busier the internet is, the slower your test will be. The busier the server you're connected to to run the test, the slower it will be.|
Since you've already replaced the modem, it doesn't seem to be the culprit. It's possible the router is causing the issue so if you actually read my guide, you'd know I said if all the troubleshooting doesn't make a difference then it's like your router is the issue.
You ask about temperature, well, heat and electronics are enemies. So any excessive amounts of heat aren't good for any electronics. Sitting in a room at 70 F (20 C) won't do any serious damange. But sitting in a room that's like 90+ F isn't good.
You mention having the router for 2 years. I've seen them come out of the box bad. It happens. Hardware fails........some sooner than others. To test, get a replacement router you know works and try it for a week. If the issue goes away, it was the router. If it doesn't, it's something else.
This sounds a lot like a congestion issue to me. You mention sharing the internet with a family. If while you're online anybody else starts downloading big files or streaming video, or doing anything that is bandwidth intensive, it will slow you down to a halt.
Do you have any siblings/kids that run point-to-point sharing applications? If you do, make them stop and you'll find your bandwidth issues will disappear.
Is your sister/daugher watching videos on YouTube while you're doing your tests? If so, then make her stop and you'll see the issue goes away.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.