Hi, if you have oscillations caused by very long leads to the power transistor, you mentioned 60 cm which
would have an inductance of 600nH approximately, then you can most likely solve this by soldering a
47 ohm resistor to the base pin, 1 to 3 cm off.
Also if possible the collector should be decoupled to ground with a 100nF capacitor (here I am not familiar
with your circuit diagram).