We would need more information about your control and process to give you specific help.
What is the complete model number of the control. If it is one of our West brand of recent years then the part number is often on the outside of the controller housing.
Is the temperature oscillating?
What are the settings for your PID parameters:
Proportioning band?
Reset?
Rate?
Is Auto Tuning active?
If oscillating how long does it take for a complete oscillation?
How much does it overshoot and under shoot?
Did the controller work properly before?
Temperature sensor placement?
Did you change something like the temperature sensor just before you started having a problem?
PID parameter notes: