you need to connect the shield of the cable to the computer case. do this with another wire, and connect the wire with a connector to one of the screws of your case.
BUT,
if your computer and the others are not well grounded, and a ground loop forms....this is bad. because current starts to flow from one grounding to the other. this is a common problem in network cabling.
also, keeping the cable closer to the ground, it makes it more protected.
also, the lenght of the cable is almost at the limit(100m). this means that from the EMI and RFI the signal arrives at the other end in not so high quality. are there any sources around the cable that can create strong EMI or RFI? power transformers, radio stations...etc....