Why is my amps bias on only one channel rises slowly to normal?

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I`m having a strange issue with a Technics SU-V40. In the voltage control amp the bias for Q413/415 and Q414/416 is 50mW, one channel is OK, the other one rises very slowly to the 50mW needed but it stops at 50mW, but as soon as I play it loud for a few seconds and then check the bias again (without it playing) it drops to about 18-20mW the slowly rises again to normal level. Also, the bad channel at high volume crackles very badly. The SVI 4002 is OK, because it does the same thing without it mounted.

I`ve attached the circuit diagram for the voltage control amp.

Can anyone help?
 

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Is Q403/4 thermally connected to the output transistors heat sink?

Yes they are. I think if they weren`t it would go in to thermal runaway. On the good channel it stops at 50mW after 2-3 seconds when I turn it on, on the bad channel I have to wait about 1,5 minutes for it to reach 50mW, than it also stops at this voltage.
 
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