A loose connection in the thermocouple wires can cause the temperature readout to fluctuate. Sometimes the temperature readout will appear normal, yet will not be accurate. Loose connections cause intermittent firing problems. Sometimes a loose connection will make the display readout freeze at one temperature.
To solve the problem, first check the thermocouple wires at all connection points. This will include the back of the kiln controller and, on most kilns, the ceramic thermocouple block where the wires attach to the thermocouple.
Pull on the wires at the back of the controller. If you have button-type connectors, it is possible that one of them has stuck in the downward position. Look at the buttons. Are they level, or is one lower than the other? If one is lower, it is stuck. This would be the cause of a poor connection. Press down on the sticking connection button until it releases and moves upward freely. Then gently tug the wires to make sure they are tight.
Check the ceramic thermocouple block for tight connections. It is possible that an over-tightened screw has broken a thermocouple lead, causing a poor connection. (The thermocouple leads are the two small wires that come directly from the thermocouple, which extends into the firing chamber.) In this case, you may have to replace the thermocouple.
It is also possible that the connection point inside the thermocouple is bad. If you have a sheathed thermocouple, you will not be able to see the tip, where the two wires connect. A poor connection inside the thermocouple can cause the same problems as a poor thermocouple lead wire connection.