Place a swamping resistor across the first audio input choke or transformer. A value of 10K to 100K will usually do the trick. That can be made into a volume control through the use of a potentiometer rather than just a swamping resistor. The value should not be critical, but use the highest value that will suppress motorboating. Check the bypasses on the audio stages.
Poor bypasses can lead to oscillation.
If there is an audio cathode capacitor and resistor that could be the problem. Decrease plate voltage in the audio stages to around 90 volts and see what happens. If it is stable there, then increase the audio bias and increase the bypassing capacitor values. Increasing the value of coupling capacitance in capacitively coupled stages will sometimes help. It seems to load the audio input down somewhat, making it more stable.