In some extreme cases, shielded or screened Cat5 could help you avoid outside interference. However, it must be installed with the correct jacks, plugs, patch cable, NIC, hub, etc. to carry the protection all the way through the circuit. Incorrect grounding of the shield can cause even more noise problems as it can act like an antenna.
If possible, I would suggest you have someone with a network wiring tester test the wire in place and see if you have a noise problem. You weren't too specific about what happens when it goes from good to bad, but assuming you just can't get data through that is definately bad.
Of course, check the simple quick things like jacks, patch cords, nics, etc. Do you lose link completely when it goes bad? Or do you maintain link but just have a reduced throughput? If you lose link, and the link light actually goes out, I would consider wire/jack/plug/patchcable. If you maintain link, but lose throughput, I'd not be so concerned about the wire unless it is at the same time or times throughout the day. Perhaps something causing intermittant noise is comming on and off and interfering.
As always there are a lot of choices, and yes IF you are picking up some extreme noise externally then shielded or screened wire (or UTP in metalic conduit) may block that out. I have had to do that only in Broadcast facilities with very high RF fields, not in industrial, but it all depends on what is affecting you.
Good Luck! It is only my opinion, based on my experience and education...I am always willing to learn, educate me!
Daron J. Wilson, RCDD
daron.wilson@lhmorris.com
