I have found a few transistors that develop collector to base leakage that looks similar to what you observe. Leakage increases at high temperatures and so as the transistor gets wamer, the collector current increases and the temperature increases and so on. The transistor should be replaced but you may find that a resistor between the base and emitter to divert the leakage current may help. I don't have the circuit so I can't say what value, but I would try 2K2.
That transistor is quite old and the processing or manufacture is not as good as more modern devices.
Cheers,