Not read everything, so apologies if repeat.
Check the wiring is correct. Modern pc lan ports are pretty good at forgiving wiring errors and working around it - eg once upon a time you'd need a cross over cable to connect 2 pcs, now you just connect a cable and they work it out.
So if you connect a pc to the cable and speed test, all looks good. The switch might be less forgiving so is actually transmitting at 100 or just getting itself in a mess so constantly throwing errors if the cable wiring is off.
I have Ethernet in my garage too. A couple of years ago I purchased a 20m cat 6 cable from Amazon. Not the cheapest, but not expensive either. Its running from the back of my BT homehub in the middle of the house to a Cisco switch in the garage/office. All worked well, but not at the speeds I expected. I upgraded to Bluejeans bonded pair cat 6 and the performance is perfect. I can’t recommend these cables enough. Every Ethernet and hdmi I’ve purchased since has been Bluejeans
I had a similar issue a few years ago in another house. I traced it to my samsung tv which had a wired link to the switch shared by my audio system. When I disconnected the tv there was no stuttering in the music. So I connected the tv using WiFi and the problem went away.
So when you are in the garage make sure you disable your WiFi and see if the problem reoccurs. While you out of range I wonder if it is just picking it up occasionally and then swapping to it. Shouldn't do but given cable seems ok seems odd.