sounds like a bad solder joint.

However, Also check the tolerance of all your resistors as well! carbon and carbon composition resistors drift over time, and thats a discrete transistor power supply so things are more critical there before itll oscillate. They also become heat sensitive as well.

So you could have a resistor in the feedback and/or soft-start circuit drifted just enough so the circuit wont fire. turning it off and on sends spikes/ripples and that can trigger the oscillation.

Just a thought.