Comment by DannyBee

I have fs.com armored cable installed for 10 years now. Different times and spools. It is all in perfect shape. They test all their cable. Something is almost certainly disintegrating your cable. If you have any spare old pvc pipe or something in the basement, hit it with a hammer and see if it is brittle (the hammer should bounce). If it is brittle, I would not hang out in the basement until you figure out what is going on.

One random guess other than gases of various sorts: If you have the lights on a lot, it's possible you have lighting that is not properly uv filtered somehow.

(It theoretically could be lots of heat/humidity vs certain types of cable jackets, but that would be obviously noticeable most of the time)

The fiber and even the armor itself looks fine in your pictures and I bet the error rate is zero. Outside of bending, i can't see how you could damage the fiber when the armor is in perfect shape. That armor will unravel if you really damaged it. The speed thing is not how fiber works. It's hard to get marginal link enough to generate retries that degrade your speed by 10%. Most of the time either you have full speed link, no link, or so many errors speed is zero. The optics almost always have rx/TX signal strength DDM you can look at

Also did you say you direct buried it in cement? If so it's not rated for that. Direct burial and concrete tight/safe are not the same thing at all, not the least of reasons being concrete is highly alkaline (ph12-13) when poured