I found 50V in the emiter of Q14 and -10V in the emiter of Q15 when the issue apears, measuring with the amp off.
Hello Fail.
As said highter in this subject , the fact you found + 50V in the emiter of Q14 and ( the + power supply) ,...and -10V in the emiter of Q15 ( the - power supply) , when the power amplificator fall in failure, is not normal .
Here is certainly the problem .
As said by Ylli , you have so , to verify the voltage of B+ and B- ...normaly indentic , when good fonctioning , at 0.1 Volt .
Certainly a problem of bad soldering , or bad contact beetween wires, somewhere , ... or the bridge with 4 diodes IN5402 , or the capacitor C20 that's in end of live and block the voltage...but in this case you have probably ,D12 and D13 very hot ...if not destroy .
If I resum , you have certainly a power-supply problem .
But you have an intermittent failure ...and this kind of failure is not easy to repaired, bad contacts or bad soldering resolved .
With time ...we can repair.
So if you found no problem in power supply , (I would be surprise) make a test at each end part of the amplicator on transistors , the differential , the driver and the power , in good fonctionning , and in bad .
And track the way , where there is a problem .
But I'm pratically sure , that you have a problem of power supply.
Can you make a photo of your ampli. , open .
Good work .
Have a good day .
Jac