Now you have a major mystery. Even cat5e cable in very resistant to interference. In the many years I have worked in this field having installed or been responsible for having others install many thousands of cable installed I have only seen 1 that was getting external interference. They had run the cable though a florescent light fixture next to the ballast.
Shielded cable is actually very hard to install correctly. The foil shield must be grounded on both end of the connection. If you do not ground it actually acts as a antenna and increases your interference. What you will see there are special rj45 plugs that have metal tabs on the side. Most equipment that is designed to use shielded cable has corresponding grounding connectors inside the jacks on the sides. You can use special couplers that let you ground the coupler if the equipment can no provide it. Still when you do it externally like this, not on specialized equipment, you run the risk of grounding loops. This happens for example when you would connect 2 buildings that the ground was not exactly the same. This is mostly a safety issue not so much a interference issues. Because this is complex and I have only had it required on telco equipment I have always had a certified electrician install it.
With the cost of the cable and to really get the grounding right I would be looking at low end switches and fiber before shielded cable if I was doing it myself.
Still this is extremely rare to see interference that degrades a cable.
Unfortunately you are at the point you need a special meter called a TDR. I have been looking for a cheap one for years for myself personally but they are expensive even used. These can actually see the interference and they can tell even what pair of wires is having issues.