Need help troubleshooting a network issue

Hi! I need some help to understand what's the problem with my setup.

I've recently deployed a wireless network solution with Unifi devices, combined with two small Cisco switches (sf-110d) and a Mikrotik router.

These two Cisco switches are about 150ft/50m apart. Once or twice a day the link between these two switches goes down (led off). All the other ports are unaffected. The switches are unmanaged and new. Unplugging and plugging back the UTP cable restores the link between these two switches.

Also, till a couple of days ago we had a TP-Link switch in one of the ends, presenting the same behaviour. It was replaced with a new one (Cisco) to discard any possible fault but the problem is still there, no matter which switch we use.

I suspect in the UTP cable or RJ45 connectors. I replaced yesterday one of the ends but it didn't solve the issue, ...last night had the same problem again. I plan to replace the other end today.

My doubt is if there's something else that can trigger this kind of failure. Spanning tree is not an option AFAIK, ie: we use non redundant topology. I still don't understand why simply unplugging/plugging the UTP connector restores the link. Is this a physical or logical issue? Any ideas?

Thanks in advance for any help.