Erratic behavior of thermocouple input
As Aaron has said I the comments, I think this is a situation for some old fashioned troubleshooting. One of your devices or connections is probably bad. Swap which thermocouple is connected to which PLC input, swap the transmitters around (assuming they're not built into the thermocouple or PLC). Swap the thermocouple locations. Wiggle the connectors. Take the bad thermocouple out and place it next to another thermocouple and see if they read the same. Etc, etc.

As you swap things around, you almost certainly find that the problem follows one device, connector, or configuration. From there it should be fairly easy to figure out what to replace or fix.