Internet connection occasionally becomes slow.

By far the easiest way to cause poor performance, instability, overheating and crashing is to install ANY third-party speeder-uppers, Cleaners, Optimizers, or Virus scanners, Bit Torrent, or a VPN that you installed yourself.

¿are you running anything like that?

Hold down the Option key while you click on the Wi-Fi icon on the menubar to open up the tools for investigating and fixing Wi-Fi issues

First is "Open Wireless diagnostics", which opens the wireless diagnostics Assistant, but does not proceed.

There are about eight different things you can do from here, but the top-level is to choose Diagnostics off the Window menu, or simply click continue, to do a quick check for Gross misconfiguration or operating problems. Your Admin password will be required. If any recommendations are shown, you should consider then seriously.

Next is to hold down the Option key and click the Wi-Fi icon as before. The screen that opens shows operating parameters of your network. Screenshot, transcribe, or photograph the results and post back in a reply on the forums. Looks like this older one.

...

researching other Routers are on your channel:

The usual way we do this with ONE Router and No boosters is the set the Router to "automatic" channel selection. Then when the Router powers up, it listens momentarily to what is already on the air, and adjusts to the least-busy channel.

it is not an accident that every debugging procedure starts with "cycle the power to your Router."

The next level we can use is to use the SCAN tool in Wireless Diagnostics to look around at what is visible over the air around the Mac. This has the advantage of showing what is seen at the Mac, which may be slightly different than what is seen at the Router(s).

this is what it looks like:

(drag and drop on Preview to see larger, or make your own)

Click on the channel column to sort by channel. This tells you the nominal channel in use, but every channel also spreads up and down the channel numbers, depending in its channel width. For example, channel 32 at 80 MHz wide spreads to engulf everything up to channel 48. You may need to look up the spectrum spread with some tables, available online.

Your raw signal level, RSSI, at -70 is not good.

right next you your Router, you expect to see numbers like -40. in office buildings with multiple Router coverage, the minimum acceptable is -65. At slightly lower than your -70, you Mac will disconnect and refuse to re-connect.

you have made an 80 MHz wide connection using 802.11ac rules on channel 44, which sprawls up and down the spectrum engulfing 36 to 48. ANY other Routers using any of those nominal channels will collide with yours. There is noise at -92 that suggests another Router might be on your channel.

You have made a connection using two antennas and only 16 patterns per signaling interval. Your hardware is working fine. but at only 260 M bits/sec transmit speed of a best-case 866 M bits/sec.

You need to be sure no other Routers are using your channel, which is the most popular 5 GHz channel.

You also need more signal for reliable, trouble-free operation.

One way users accomplish more signal is to eliminate building materials that occur in the line-of-sight between Router and Mac. Another is to reduce the distance between the Router and the Mac. Often a Router might be plugged in a basement area, far from your devices. if your signal comes in by cable-TV cable, consider moving the Router to a different cable outlet or adding a new one.

There are lots of other solutions as well, just ask.

EDIT: I made several mistakes interpreting your parameters, now corrected.

VPN software can be very intrusive. and YES, it has been seen to cause problems even when installed but not activated.