I have an LNK-304GN design that has been shipping in a product for over 10 years without issue. It supplies 18V to a varying load that averages 55mA, but can see steps of about 3X that current. With no changes to the load design or the power supply, the output of the power supply has suddenly begun to oscillate at about 5kHz when a step change in the load occurs. It does not appear that any supplier's components in the circuit have changed. i am baffled as to how to troubleshoot such an issue. The schematic is attached, and as you can see it follows the design outlined in the chip spec and the App Note. And again, it has had no issue for nearly 10 years. Any suggestions as to where to start looking for the source? Thank you.